Do we have to use a potion, elixer, zapper,etc?

Thank you very much. I will search for it.

You use a Nightmare Potion, but you can also get it by zapping as well, but it's random what species you'll get.

You can use a Nightmare potion or if you're really lucky, both zappers are known for giving out nightmare pets.

for future reference, if you check out the useful help threads and go to the FAQ, all info on how to change your pets' color is in there.