Well, then tweak the wording a bit. It would randomly go to any current dye kit colour available for that pet. Fixed.
I reckon it's quite a good idea, especially if the ratios were tweaked so dye kits that cost more would be rarer, with the Mystery DK exclusive colour(s) being the rarest.
It would probably work better if there were more existing DKs though. For example, if you Mystery DK'd an Omni, it would be either GaS or the Mystery DK exclusive colour(s). That could put a spanner in the works.