Name a sheep "jeb_" and look at it.
In 18w01a it looks like the method cid.a(afc, float, float, float, float, float, float, float) tries to compare the string "jeb_" with the returned text component, which likely always fails because the classes are different.
MC-124997 A sheep named jeb_ does not change color
MC-126820 Naming sheep jeb_ does not make the sheep rainbow-colored
MC-127241 Naming a sheep "jeb_" doesn't it make it change colours