The toolset - Near Infinity, that is - gives limited options when it comes to creature colors. If you want to change the hue of someone's skin, major or minor color, the tint of the blade, the rim of the shield and so on, you have about 250 colors to choose from. This is not as many as it sounds - a lot of the colors are glossy and only good for metal or leather, others are too dark or too bright to be useful anywhere. Reds are especially scarce - there are only three real red colors. I found a way to infinite colors, though not glossy colors. If you want more metals or leathers that gleam as the creature moves and turns, I can't help you.
To change a creature's colors you need to either have the engine cast a spell on it or make it equip an undroppable item with Instant/Permanent Natural/Nonmagical Self-targeted effects. You can randomize them if you want. To do this en masse, for whole categories of creatures, use Weidu to put items on them. (You can read my recent posts at Spellhold about this.) Put Set Color Glow Solid effects here and pick the creature part you want changed. Here you can draw from the whole spectrum at any brightness and saturation. Unless you want to imitate glows from Tron, put them both in the bottom of the table.
This produces a good linen-hue yellow not found in the regular colors:
Brightness controls how glow-like your color is, saturation - how much it acts as an overlay. If saturation is very low, nearly gray, the color will be a tint of the regular color of the creature. In this way this function is somewhat useful for skin - as a solid color it is too flat, especially when there is a lot of skin shown: