By now, gang-colors could be any combination with black
Generally worn by members of the same gang in order to identify each other, gang-colors can be dangerous if you aren't wearing the right ones.
You are wearing black jeans and a purple shirt. You get off the bus in a neighborhood where there are many gang members. Unfortunately, they wear black and yellow. If you look at all threatening to them, good luck. I hope you don't get shot. Of course, depending on the area you live in, your nationality may play a part in how threatening you appear to the gang members.
Gang-colors have been put on the list of banned clothing articles in schools in many large cities.