The colour of the pet moves with the species change providing the pet you are morphing into has that colour. So a Woodland Kau could become a Woodland Hissi.
There is however a workaround to get a pet of any colour through the perk. If the species you are morphing into does not have the colour your pet is currently painted, then you can pick any colour you want.
So the Hissi for instance does not have Sketch as a colour. So if I morphed the pet I wanted to change species into a Sketch Acara, then started the Premium Perk and picked Hissi, it will tell me that the Hissi doesn't come in that colour. I can then pick ANY Hissi colour I want. Including lab only colours like Chocolate. That's how I got my MSP Poogle.