On Marapets users can get a hold of items called "Costumes" which can change a pets colour, the four basic colours on Marapets are Red, Blue, Yellow and Green. A user can dress there pets in the costumes in order to change there pets colour. Marapets have a total of 111 colours in total with an extra two added every month. Marapets currently has 25 more colours than Neopets.
If you want to change your marapet's colour you can use costumes and potions. Though there is a shop of potions the price can raise from prices like 67900 MP for one potion. To see what colours your Marapet can change into go to Gigantic Paradise and click on Pet Colours (on the rainbow).