Hi,the predominant ground color in bulls is yellow,so the correct term for a genetic mutation blocking the production of that pigment would be "axanthic".that being said,I have seen some examples from other people's breeding efforts,that would suggest that this genetic trait also works on the "red" pigment seen in some bulls,(Kingsville & "Crumbley" )however,I personally would still consider the correct term to be "axanthic".