You might be able to come up with some interesting monsters if you look into what each color can represent. Some examples would be red for anger, green for sickness, blue for sad, white for scared. Things like that might help give you some ideas.
The opposing side wants to steal the worlds colors and establish grayness, which also means unlively, dull, mechanic - briefly, they steal the joy in life. I don't think it'll be right to make them particularly colored. White, black, gray, that should be their traits.
Not all opponents are part of the gray hordes, so there can be colored opponents as well. E.g. animals in the wilderness, or random dungeon inhabitants.
The final version of the project (if it ever gets there) will have 7 differently themed locations to explore, and I think at least one, better three should be purely gray, the others can be more or less colored.
At the moment I'm trying to narrow blurry ideas into a design which I can implement.
The arch evil is the Gray Lord, but the player doesn't necessarily have to kill him to win. The player just need to protect the colors of the world which he can do by retrieving 7 artifacts (the rainbow jewels) which the Gray Lord has stolen.
Now I need some servants and minions of the Gray Lord.
Gray in this game is a bit like undead in classic RPGs, void of life's joy, still living.
The Colour Out Of Space?
Maybe my English is insufficient. What would that be?