So do not expect to be unable to tell the door is actually white, even if you are imposing a red door on top; if you cannot tell the color of the door after imposing, that is likely a mental problem that tulpamancers cannot help with. And, as stated previously, it is indeed a skill separate from tulpamancy in particular, it's just often used by tulpamancers because it is nice to be able to interact with the tulpa in that way