this is Julian Sprung's reply to me:
It is true-- you can see it in the wild. When A. clarkii or A. percula are in Stichodactyla (carpet) anemones they have lots of black. When A. clarkii is in Heteractis crispa it is brown, when A. percula is in Heteractis magnifica it looks like the typical ocellaris (albeit a very orange one).
There may be some genetic component in the case of percula. A. percula in S. gigantea (typical "onyx" clowns) are found near shore, in seagrass, and in lagoons while A. pecula in H. magnifica are found on offshore clearwater high energy reefs. So, they are somewhat separated. It would be interesting to take offspring of each and put them in the opposite host to see what happens.