Choose the color you want and give your pet the potion that corresponds with that color.
Thank you. I was not sure is there was different potions to do that.
Better yet, if the color comes in an elixir, even if it is for a different species, use that elixir on your pet and then transform them into a blob. In many cases, that is the cheaper route. The newly transformed blob will retain its previous color, as long as that color is available in blob.