Our neighbor had a bull next farm over in the pasture. If I wore yellow or white t-shirt I could cross the corner of the pasture. If I wore red or orange t-shirt he would start coming. I never tried green coz I didn`t have a green shirt. A white horse got out from an adjacent pasture into the bull`s pasture. The bull gored him. Our Morgan stallion would trot right past the bull in the fenced-in pasture and the bull didn`t even charge. If our white Arabian trotted by the same area, the bull would hit the fence. If bulls are color-blind, they don`t know it coz my fast running legs as a 14-year old tell me otherwise.
I think it`s the type of target, the scent, and the color. Did you ever try to run away from a bull with you wearing different color-t-shirts? I did.
I think empirical data from a farmboy outshines any scientific theory.