I absolutely agree, but has anyone ever done a study on how much influence face colour has on number of lambs sold?
Yes they have. I spoke to the late Ian Cunningham not too long before he died, and he recounted how he compared dark-faced and lighter-faced Mules at Edinburgh, and found no difference. I think Redesdale EHF might have obtained the same results.
Most wise shepherds would agree, but it is a fact that good stockmen will much prefer to work with stock with a pleasing phenotype, so long as it doesn't detract from their productivity.
And the fact that well-marked darker-faces bonnier Mules make a significant premium over their lighter-faced cohorts is testament to that.
But I suspect that breeders of Mules who can produce those with good heads, are also able to instill more productive traits into what they breed. Other things being equal, the rams from breeders of Bluefaced Leicesters which pass on both bonny-heads and good productive traits to their offspring will invariably sell at a premium over rams that pass on plain colourless heads to their offspring.