A red rag to a bull
Right, this has been bugging me for ages (well, as long as the new Vanish Oxiclean advert has been on telly).
I have gotten bored of searching Google, because lots of people use "red rag to a bull" as a turn of phrase.
However, I'm pretty sure that bulls only see in black and white and that if you held up any colour sheet and waved it about that it would get aggravated. The red is just for effect.
In which case, the Oxiclean advert is all wrong, because it would still be agitated by a white sheet being waved at it and the red spilt on the trousers would make no odds.