They don't hate it. This is a common misconception thanks to the Spanish bullfighters and the use of their red capes to intimidate the bull they are fighting in the ring. Bulls and all cattle in general are in fact dichromates, and only see in blues and yellows, so they only respond to movement, not colour. The red colour was used because it made it easier for the audience to see the bullfighter, not for the bull to see the bullfighter. It is also tradition in the Spanish culture to use red in festivities like this.
Bulls become enraged by the movement of the bullfighter's cape, not by its colour. The red colour is probably traditional because of the obvious connotations with blood.
Wearing red into a field containing a bull is no more dangerous that not doing so, but I wouldn't reccommend it.