It's important to remember that the terms "dominant" and "co-dominant" refer to the way a mutant gene interacts with the wild-type ("normal") gene, in the way that we use the terms. It doesn't tell you anything about the way two different mutant genes will interact with one another.
That doesn't mean that one mutant gene couldn't totally "mask" another mutant gene -- for example, I would hazard to guess that most mutations will be totally "masked" by the homozygous lesser mutation (solid white snake). (Don't know for sure, but I'd bet they will.) I think it's actually kind of amazing, in a way, that the various ball python mutations seem to have all the cool interactions that they do ...
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