Yes fibromeletic (may have misspelled that ) is dark skin / flesh
White cap did have a rumpless Gpa but he didn't inherit it, I do not know if that gene can be carried and not shown though. I have one in an older group a chicks that inherited tufts even though mom had none (or dad) Gpa had small ones.
I have a chick that was hatched rumpless this spring, and I don't have any others in my flock. So does that mean it is likely this particular one didn't develop properly? She did have quite a terrible time with pastey butt in the beginning, but she is a healthy and happy little thing now.