Every color can be described with three basic numbers – hue, value, and saturation. The hue is whether the color is red, green, or blue; the value is darkness vs. light; the saturation is how much “color” there is in the color (a really bright red has high saturation; a faint red that’s almost white has low saturation; both of these would have the same hue and value).
Now, when I look at things qualitatively, especially when thinking about mythopoeia, I tend to think of them in terms of colors. For example, in Orbivm, there’s a lot of color imagery – Lavinians=red, Sidhe=green, Marauders=blue, for starters – and that’s all intentional. Somehow those colors just seem to fit with those civilizations.
These are all different hues, clearly, but basically the same value, somewhere around the middle (though there’s some variation). This because good is white and black is evil, traditionally. There’s not really any civilization in Orbivm that could be qualitatively considered as “black” or “white”… they’re just different hues.
There’s a reason for this, I think. If you have one civilization that is pure black and another that is pure white, then their average saturation is zero. So it seems out of place for there to be a red, or green, or blue group; the fantasy world has been cast in terms of black and white, and there’s no room for color.
Which isn’t to say that black and white fantasy is impossible or pointless. The last few short stories I’ve written have been very dichotomous – light versus darkness, and all that. But when you do this it’s necessarily simpler – with only value, you can do much less than with hue, value, and saturation.
What’s hard to do well, I think, is combine light vs. darkness and red v. blue v. green. It requires forcing each civilization, which was originally just red, or blue, or green, without any real moral alignment, to pick a side. What you can’t do, as someone on the Wesnoth forums tried to, is leave the RGB valueless and just add two more groups, one black, one white. Doing so is conflating hue and value; they’re simply different things, and have very different implications for a fantasy world.