On Marapets users can get a hold of items called "Costumes" to dress their pets and change the pet's colour so it gets added on to their Transformations collection. The four basic colours on Marapets are Red, Blue, Yellow (the first three are the primary colors) and Green. Marapets have almost 6,000 colours in total with an extra two added every month. Marapets has more colours than Neopets.
If you want to change your Marapet's colour you can use costumes and potions. Although, in the potions shop, 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).