Hot water bath will allow more 'stuff' to be pulled, often that more stuff results in yellow color. I found quickly that I could get some very clean first pulls with a clear layer of naptha.
So no, the naptha does not need to turn yellow.
My ego is insane, but I'm alright
The path of excess leads to the palace of wisdom. -William Blake
Lavos is a fictional character, a dream inside a dream. Don't take what he says to be true or representational of reality in any known form. He is inspired by pure fantasy.