Looking at my artwork, you may notice that I usually enjoy the use of similar colors in a piece. For example, a color scheme of yellows / oranges / reds, or maybe light blue / dark blue / purple. But even with similar colors, I always try to add variety and variation in their hue to keep the eye interested. And doing that also adds realism to your piece.
Color choice helps add mood to a scene as well. In fact, on larger productions, some companies have people who are devoted to picking the color palette for a particular shot or sequence to make sure the overall look conveys the right feel to the audience. Color theory is a vast subject, for this tutorial, I'm just going to pick one small lesson that I was taught the first few years I attended art classes with Canadian Wildlife painter Renate Heidersdorf. I was very lucky to get a place in one of her classes when I was 6 years old, and I continued going to those classes until I was 23.
Every year in the fall, classes started out with us doing a nature
of some kind. When I was like 7 or 8, we did the following, first, open
the ink that was called "Green". Place it on the paper. Observe the
color. Probably medium brightness, not quite neon, but very generic.
Then in the next 20 minutes we had to mix on the paper 20 different
Green with yellow mixed in. Green with blue. Green with brown. Dark
light greens, Lime greens. As many greens as we could find.
The purpose of the exercise was to realize that just because this ink was called "green" does not mean it should be used to paint anything we know to be "green".
Once we finished this exercise, we then started out by painting a nature scene of some sort. And of course, we were expected to use lots of different greens, different greens for different trees, grass and plants.
Now, it may seem odd to have small children follow a rule like this, "No premade green is allowed in your painting". But in fact, it is not a rule as much as a method of breaking the rules. When we are small and are holding our first crayon, we have total freedom. If we want to make the sky green, we can make the sky green. I mean, why not, if that's how our minds see the sky. But soon afterwards, we start looking to our parents for answers to our questions. Such as, "what color is that tree?' "Well, it's green". "What color is that grass?" "It's green." "What color is that car?" "It's green". Pretty soon, we have all the answers we need, all of these things are now categorized, they are all green. So no wonder when a person starts painting, they immediately reach for that generic green to paint their grass and trees. After all, they're all green, this paint jar says green, so I should paint it green.
This is the difference between painting what you see and what you know. In the book "Mass" by John Harris, he explains how he had a teacher who was convinced that is was more important to paint what you saw then what you knew. As in, forget that you know what that object is, paint the lights, paint the darks, paint the color, paint the shapes, and eventually the object will just take care of itself. Try this as an exercise sometime, find something and try painting what you actually see vs what you know the object looks like. Your observation skills will multiply in no time.
And of course, one of the first things you'll notice is that "hey, the green in that leaf is not the same as the green in this leaf." Or "This shadow makes this green darker than the green of this leaf in the light." All of a sudden you see how the object really looks outside of what we expect it to look like.
The Natural World
Lets take a few shots of the natural world. Here's a patch of ivy, notice the huge variety of greens, some leaves are light, some are dark. Some are in shadow...
Even upon close inspection, the leaves themselves are different colors, with veins and spots and other detail, many, many different greens.
And here's a lovely garden scene, plenty of different greens there,
I've picked a few of the colors and placed them on the right side for
you with Photoshop's color picker.
Lets Make Some Grass
Following these ideas, lets make some grass. I used the shaghair plugin for this from digimation inside of 3DStudio MAX, this plugin is now called HairFX and is available here. Below is a picture of my grass, with a generic green color. Ok, looks good, but there's something missing. The eye sorta gets lost in the grass, since it's all the same bland color.
Fortunately, most plugins of this kind probably gives you users some options as far as color goes. For example, you can use a map to specify the color of the blades of grass. Here's a map, imagine this projected from above the grass. When a blade of grass grows, it chooses it's color based on the color of this image at the position the grass is growing. So if it grows from a pixel that's black, the grass itself will be black at that point.
And here's the map applied to the grass. Notice how this adds variety and ends up being more pleasing to the eye, as well as more realistic.
And here's some real grass, notice the variety in greens...
Of course, depending on what you're trying to do, single color grass may be the goal, such as a more cartoony kind of grass, but if you go this route it should be a conscious choice based on a style. A common mistake when you just start out is to make everything the same color, not because you want that but just because it didn't occur to you to do anything else.
Along these lines, I've also written a few scripts for 3dsmax that can help. For example, the script RandomMaterial, this lets you apply several materials randomly to a selection of objects. So maybe make a rock texture, then make a second texture that's similar but darker, another one that's lighter, one that has more yellow, etc, then select maybe 100 different rocks, and use this script to assign these materials randomly to the rocks. Now, the eye won't just see the same identical rock, but slight color variations in the rocks.
Color variation is an important step in understanding how the real world works, and can be used as a powerful tool to add richness to a scene.