As I understand it, poisonous things are often orange.  But, then, plenty of things that aren't poisonous are also orange.  If you can mimic a poisonous thing and convince other things to not eat you, bonus!  Producing poison is actually rather costly, so just being a scam artist is a cheaper way to survive.  Of course, this only works if actual poisonous stuff outnumbers scam artist plants/critters.  If the percentage relationship is reversed, the whole thing falls apart because other things conclude that "You're probably safe to eat, since most other things that look like you have proven to be safe".