I don't think it's racist in the human meaning of the word, more what they are used to.
Our Hebridean sheep were horrified one year when they realised they were expected to mate with a Texel tup. Most came round again but finally gave in. Our neighbour keeps Texels (so ugly, poor things) just through the fence, so I suppose that knocks my theory on the head.
But, one ewe was perfectly happy and she was the one who had fostered a Texel lamb the year before.
Previously, my flock had accepted a Texel ewe who joined them to foster a pair of Jacob twins whose dam had died.
So nothing definitive really, just a bit picky.
My hens just mix willynilly. They accepted a white cock last year (hens are mostly brown or black) with no issues