I think the biggest problem with reducing the color palette for posts is that is a 'hard function' in vbulletin, so would probably be a long time of hand modifying code--ie, not happening. The other option of completely removing color for a post text option is more easily possible, but I don't remember exactly what that does to existing posts (I think nothing, but been a while since I've looked at these options inside my admincp).

Not great solutions either way. I'm all for requiring people to cut and paste to notepad first to get rid of colors and formatting before pasting into a post.