Kauvara's Magic Shop
Morphing Potions are some of the most valuable items in Neopia. Potions can either change your Neopet's color or change your Neopet's species, or they they can do both. For instance, a Yellow Elephante Morphing Potion will only change a Red Elephante's color. However, that same potion will completely change a Blue Eyrie. Potions do not come in every possible variety, but there are dozens of different ones out there worth varying amounts of Neopoints.
All potions are made by Kauvara in her magic shop, however occasionally tainted potions make their way into her shop and can have quite sinister effects on your Neopet. Created by Dr. Sloth and known as Transmogrification Potions, these vile concoctions will turn your Neopet into a mutant. Perhaps the most famous of these potions is the Draik Transmogrification Potion, although there are many popular ones out there with new ones being released all the time. Only these potions, the Lab Ray, and the Rainbow Fountain can make your Neopet into a mutant.
If all you desire is to change your Neopet's color, make sure you check to see if the Paint Brush required is less expensive than the Morphing Potion. If you want to change your Neopet's species, but don't like the color of the potion, then find the least expensive potion for that species, then buy the paint brush you want. It may be less expensive.