Herminny, you're true, we mustn't think every colour mentioned in books is a symbol. But there is a lot of symblolism anyway, check it out at French site: http://www.lapensine.com/index.shtml - moderators, have mercy, I'm not advertising this site, just giving references .
I have delat with green/red paradox already in another topic on this forum, but can't remember the name of the topic
It was sth. like this: if green represents evil, Slytherin, blah, blah, then why does Voldemort have scarlet eyes. This is supposed to be a warm colour but JKR always describes his eyes as cold, pitiless, merciless, ... her colour symbolism is much deeper that just complementary colours like red & green in the colour circle. I think she has one colour for a lot of symbols