Bulls, along with all other cattle, are colour-blind to red. Thus, when bullfighting, the bull is likely irritated by the cape’s movement as the matador whips it around, rather than by the muleta’s colour. In support of this is the fact that a bull charges the matador’s other cape – the larger capote – with equal fury. Yet this cape is magenta on one side and gold or blue on the other. (http://bit.ly/2nzJdpL)