Just curious how do you change your pet's color? Is it with a potion, also how do you get them?

depends on the potion.

Elixirs restock in the apothecary main shop. The rarer potions, bloodred, reborn, nightmare also sometimes restock in the token shop. They also restock in the apothecary though it's rare to actually get one. I've always just bought them from token shop restocks. In order to get a glade potion you'll need to get a ornate floral pitcher and fill it with water from Peka glade. It's only open at certain times.

Galactic, Hydrus, and Nostalgic are via quests.

Note, in order to do a Hydrus quest, you'll need to have assembled a uncharted land map to get access to the Omen Islands.

Storm potion is through a random event.

You use a potion or an elixir, you can get them from the main shops (Apocalyptic Apothecary), the token shop and many other ways around the site. Some colors are only available through quests, like galactic or hydrus. You can also buy potions and elixirs from other users.