Painting your neopet could be a hard decision. Lot's of users would paint their favorite neopet with the paintbrush they have in their inventory, but what if your neopet looks horrible? Like, an asparagus pteri? Even though asparagus pteries (if that's how it's spelled) aren't real, just imagine. Would that be a pretty picture?
Lot's of Neopets help sites offer images on what a certain painted pet looks like. If you are unsure or uncomfortable about painting your pet, you should go find a website that does this. If you think your neopet looks the way you didn't expect, try one of your other pets or trade the brush for some neopoints or find someone who's willing to trade paintbrushes with you.
There are lots of paintbrushes and lots neopets. So, basically that means it's a big combination of neopets. And since paintbrushes are expensive, you might to make sure that the pet you want to paint is the pet you want to paint.