Color Coded Characters

Last year in Game Design Workshop, for the first Paper Game, my team created a spy themed laser gun board game.  Each spy was given a color, and as such things usually go, we settled on Blue, Red, Green, Yellow, finding success in Google's logo design.  What makes it such that these four colors represent basicality?  The inclusion of Green instead of Orange or Purple a consequence of being complementary to red and a combination of the Opponent colors Blue and Yellow?

That's not to say that Green does not fit into the team.  Look, he's just as good a spy as any other:

I recently worked on another set of color coded characters, and as usual opted to assign them as hair colors.  These one's for Pop Quiz, a CombiForm math game: