If I use a potion on my pet for the achievement and don't like the outcome can I change it back to the original color with an elixir?

Yes. Say you have a Cream Kumos now, and you use an Arid Legeica Elixir. After use, your pet will be an Arid Legeica. If you want it to be a Cream Kumos again, use the item called 'Cream Kumos Elixir'.

yep! i agree with Blir. i have done it before