It’s hard to say. The historical person, St Nicholas of Myra, was probably of a lighter complexion, given he was a native of Greece.
However, in traditional Catholic art, he is depicted as anywhere from pale-skinned to dark-skinned depending on the location of the images (Bari in particular has many paintings of him with darker skin, possibly to show that they were cool enough to be able to attract a ‘foreign’ saint).
So given that even the person who is the origin of ‘Santa’ is depicted with many different skin tones, I’d say Santa can be whatever skin color people want him to be.