You can buy paintbrushes to have them be different colors, fairies to give them different stats, and weapons so that they can fight with different powers in the battle dome. These differences may not always appear on your pet view screen though and are only visible when actively using them.
You can buy so many things from the shop to customize your neopet. First, buy new clothes for a good looking neopet. You also know that there are so many choices. Let us go to a neopian bazaar and see different things to customize your neopets. Magic shops and art centres can help you in your quest.