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.

No. Bulls, let alone all cattle, do not have red receptors that us humans do. Cattle lack a full spectrum of vision, thus are only able to see colours that include the primary colour receptors of blue and yellow. Grazing animals are most attuned to yellowish green and bluish purple hues, and can possibly see some green as well.

To say a bull hates the colour red is wrong. A Spanish fighting bull only responds to the rapid movement of the cape, not the colour of the cape. The colour of the cape is made so that the audience can see what the matador is doing and is only to excite the onlookers, not the bull himself.