Shading with colors is not an easy thing. All those hues with different saturations and brightnesses can make you dizzy! Black and white, in this case, should be much less complicated, but that's not true at all. If you want to learn how to shade very dark and very bright objects, especially in terms of living creatures and realistic lighting, keep on reading!
A Little Bit of Theory
As you have probably heard, white objects are capable of reflecting all the light hitting them. Black objects, on the other hand, are unable to reflect any light at all. But if it were that simple, we wouldn't need to shade these objects at all! A white ball would look like a white circle, and the same with a black ball.
We all know that's not the case, so we shade these balls anyway. The most intuitive method is to make the shade brighter in the light and darker in the shadow. It gives us a proper 3D form, and may look all right as an element of many styles, but is it how it works in reality?
The Secret of Black
The premise of form-emphasizing shading is that an object is hit by various amounts of light depending on how each part is located towards the light source. The more light hits the area, we think, the brighter it looks. Therefore, we make the black ball black in the shadow, and then we make it gradually brighter as it's touched by light.
However, it's not really about light hitting the area—it's about how the area reflects it back to our eyes! And since black doesn't reflect anything at all, it can't reflect more light as it gets illuminated. If something becomes gray in light it means it was already gray!
The general rule is that perfect, 100% bright and pure light reveals the "true" color of the object. When light isn't that good, or it can't hit a part of the object, the color gets distorted. The true color of black is... black. It doesn't reflect anything, so it doesn't care about the amount of light hitting it (or not).
How Do We See Black?
The truth is we don't. That's the definition of black—we can't see it. However, none of the objects we observe on a daily basis is truly black. Dark surfaces usually reflect 15–20% of light, but they still do it. This dim light is what we see when looking at a dark object.
There are a few ways that let us see "black" objects:
This is the most basic one, but it's not the most popular in the case of dark surfaces. It applies to situations where the object is dark even in full light, but it still has certain hue and saturation.
To achieve this effect, first define the brightest version of this color. Then you can shade it normally, bringing it to almost black in the shadow, but not any brighter than that base in the light. Don't compare it to more contrasting elements of the picture. If a material is supposed to be dark, make it dark!
This kind of reflection is easy to recognize, because it moves when you move. This is that "glossy effect". You can imagine a matte black material that has a thin, transparent layer on top. That layer stays invisible until it's given something bright to reflect.
To achieve this effect, start with a black (or very dark) material. Then use a hard brush to paint a reflection of the light source—the smaller, the better the effect. Pay attention to other elements in the environment that reflect light very strongly, like a white floor. You can use a soft, round brush to reflect such an area on the black surface. The brighter the reflected object, the brighter the reflection.
The color of that reflective layer is important to determine what and how it's going to reflect the environment. A white layer will be able to reflect every color, though they will be quite dark unless they come from a light source. A red layer, on the other hand, will react to a white light by reflecting red only, and it will not reflect green or blue.
This is certainly the most popular kind of reflection when it comes to black materials. Once again imagine the thin, transparent, glossy layer, but this time make it not so perfectly smooth. The reflection is going to be quite diffuse!
To achieve this effect, shade with a scattered or textured brush. Remember to leave blackness in the shadow and to treat these reflections like any other specular reflection in terms of color and brightness!
This one is the most fascinating. Interestingly, it doesn't have anything to do with the pigment. It's a structural color, which means it comes from special properties of the material. In short, the light reflected from the surface interferes with the light hitting the surface, which modifies the signal. This interference may be constructive (creating a color), or destructive (canceling any color that should be reflected and resulting in black).
The power of iridescence lies in motion—such a surface changes color depending how you look at it. It can't be simulated perfectly on a still picture, but we can successfully use it to make a dark surface brighter.
You can use it as a specular reflection, matte or not. Iridescence lets you use colors that have nothing to do with the environment. When shading, shift Hue drastically with every level of Brightness, and you'll see something beautiful! Magical as it may look, this is completely realistic and happens a lot in nature.
The Colors of White
All the previous tricks can be used here as well, but white brings us other problems. How dark can white be in the shadow? And how is a shaded white ball different from a shaded gray ball?
White is so hard to paint because we know it should be bright, but at the same time we want to shade it to give it a 3D form. Is there any compromise?
The Colors of the Environment
White reflects everything that can be reflected. In perfect shadow, white is black, because there's nothing it can reflect. However, in most pictures, perfect shadow occurs only in the crevices. Anywhere else, ambient light is present, filling and brightening the shadows subtly.
This ambient light, no matter how weak, is always perfectly reflected by white materials. If that light is only 10% bright, a blue material may reflect 3% of it, while a white material will reflect it all. Therefore, it will always look brighter in the shadow than any other color.
The tricky part is that white reflects all the colors, even if they're not emitted by a very strong light. Therefore, white is rarely white, or even gray. Put it on grass, under the sky, and it will be covered with a green-blue gradient, with the white part only visible in the highlight area.
This also brings us to the issue of white balance. Our vision system is based on contrast—we don't see something because of what it is, but what it isn't. In the language of our eyes and brain, bright is the lack of darkness, and blue is the lack of yellow.
Because of this, everything we see is relative. Your computer may say that those are the same colors, but your eyes tell you something different. Do your eyes lie? Maybe, but in this case you'd need to say they never tell the truth! Our vision is based on illusions, and computers are lacking this knowledge. They're too objective.
We say a color has a temperature. We say it's cool if it's bluish, and warm if it's yellow or orange. The truth is that for our eyes a color is cool if it's more blue than its neighbor. It doesn't need to be bluish at all, if something placed next to it contains even less blue.
This also applies to white. You may try to make it as neutral as possible, but it will always seem cool or warm, depending on its environment. A neutral, 100% bright white will look warm when placed next to a blue-tinted white, and cool when in the company of a yellow-tinted white.
It's very important to accept this fact. Don't ever treat white as a pure, non-saturated color. Pay attention to the color of the light source and color everything accordingly—white included. In fact, it's better to choose between a cream white and snow white, leaving neutral, 100% white for highlights only.
Subsurface Scattering (SSS)
It's not always true that white reflects everything. Some white materials are translucent—the light comes into them, but instead of being absorbed, it gets reflected inside the object, making it bright under the surface.
This light isn't brighter than the one reflected directly, but it does magic to parts in the shadow—it increases their saturation and shifts their hue toward the color of the light. It is especially useful for white organic materials. It's the reason for the subtle difference between them and plastic of the same color. Even white fur uses this effect!
To create this effect, don't use a darker version of warm/cool white for the shadow. Instead, make it overly saturated and relatively bright (especially in the terminator area), with the temperature of the light.
Over- and Underexposure
There is one more aspect of shading black and white. It applies mostly to what cameras show to us, but our eyes aren't immune to it, either. When a lot of very strong light is present, the midtones of shading are literally murdered, consumed by the growing highlight area. When this happens, even black can be shaded with white!
Another side of the same situation is when there's not enough light. Highlights disappear, and shadows consume bigger areas. In this case, white can be shaded with black.
Both these situations are generally not desired in photography, because they kill details. It's characteristic for over-exposed pictures that they have big areas of white shades, and it's the same with under-exposure and black. So if you find this in your picture, this is a sign you've got something to fix!
A Practical Example
Let's see a quick example of using this theory in practice (you can download this file in the right sidebar).
I've sketched the creature, and then added lighting and base colors. Dark blue is going to be my base for black, and dark orange for white.
I've sketched the creature, and then painted a base for its body—a Clipping Mask for the future layers. Notice that it's dark, but not black yet.
Then I've added a New Layer and painted shadows on it. I've set the Blend Mode to Multiply.
I've created a New Layer below the lighting, and painted a base for white patches. Notice it's not dark white (gray), but dark orange. That's because I've decided to use warm white for the fur.
Now I've used a dark cream white to paint a sketchy fur. If you want to learn more about painting fur this way, see this tutorial.
Now I only needed to gradually increase Brightness and decrease brush size.
In the last, near-white phase, I've added a hint of blue and green, depending on the imagined environment.
To make the fur translucent I've added some orange into the shadows.
I've picked the color of the base with the Eyedropper Tool (I). I've increased its Brightness and added very subtle lighting.
Then I've continued, making the brush smaller and brighter with every step. The more matte the fur is, the fewer of these steps you should take. The final effect should be the result of your decision, not an accident!
I've decided to make the fur glossy, so I made it reflect the environment's colors. A small brush is crucial here!
Horns, antlers and teeth tend to be smoother and brighter at the tips because they're worn out. That's why I've decided to make a mix of glossy white and matte black for the antlers.
Notice that every "pearl" of the antlers is shaded separately. That's the secret of creating a texture!
A small, hard brush is the secret of glossiness!
Finally, I've added details.
"But it's not black and white, it's blue and yellow," you may say. If you want to see "real" black and white, look down. In nature colors don't exist as we imagine them. They interact with each other, so they're never pure. By treating every color as a separate entity you go away from realism, which isn't bad as such, but if your intention is to paint realistically, you shouldn't ignore it.