Since the Win Model 21 used a high temsile steel, there was/is no need for case coloring. Case coloring results from the need to harden parts made of low tensile steel material. It is/was somewhat attractive but for todays modern receivers is not really needed and most case on modern guns is not due to heat treat. Spanish and Brit guns may be an exception where they are using historical methods.
CSMC will case color a Model 21 receiver.-Dick