Still, since it is a historical game it is a shame not to paint proper colours. Why not get into 16th-17th century? Then uniform colours still varied and one formation of the Swedish army under Gustav II Adolf in 1630:ies was named "The yellow brigade" for a good reason!
Nowever, french is not just blue! The Imperial guard where usually blue coat, white west and trousers but the coat had red reverses I think on some occasions. Hey, you should be able to search the internet for Napoleonic unifrom patterns!
But please, paint your units in historical colour and save me from crying, will you?