I do beleive the tan limbs Klaatu exists because I have a few. I wouldn't really consider it rare though. I have one too with mixed colors for the limbs. Some being gray and some being tan. I really don't think it's because of age. I beleive that the factory where they were made they simply had different colors for some limbs for whatever reason.
Now there are countless paint variations on many figures. Some used tan for the the skin color and others used a more vibrant color which can only be described as skin tone. I have a Luke Bespin that has a tan face and the other skin toned hands. Looks kinda weird.
How about the IG88 gray and siver versions? I remember as a kid I had one of each somehow and didn't like that they were different. Wanted them to be the same. I have always been into army builders and having several of a figure to use as soldiers or whatever.