You can get potions, which will turn your pet into another colour - here is a list. Also that website lets you see all the different species/colour combos which is hella useful haha.
There are some colours, namely nostalgic, galactic, hydrus and steamwork that you have to do other things for.
Hope that's helpful!
Thanks! Is it possible to change a pet species? I zapped my active and immediately regret that decision... I wish he was dragarth again!
You can change your pet species with elixirs which can be found in the shop Magic Box (as well as a rare chance to get potions) or from certain quest rewards.
The list of all dragarth elixirs can be found here which have all basic (and bloodred) colours as well as common.
Hope that helps! :)