I think a general 'No Spam' policy would be best. Simply "If you're 1/2 filling the chat window with uselessness... you're spamming, so stop it!" That way it covers all the afore mentioned spam related issues as well as anything that may arise in the future
As for colors, different people will see it differently based off hardware or simply perspective. I think, as long as people are polite about it there's no reason to police that particular aspect.