From PinataIsland.info, the Viva Piñata wiki
Every species has three color variants which must be discovered through various means. Feeding a piñata certain items will change their color. For example, if you feed a turnip to a Whirlm, it changes color to a purple Whirlm. Some species have to eat more than one item to change colors.
Once you change the color of a piñata, you can't change it again. So, to get all 3 variants, you have to have 3 separate pinatas, and change each to a different variant color.
Every time you discover a new variant you will gain four petals of experience.
Note however that some variants can reduce the base value of the piñata by a whole lot. For instance, all 3 of the Eaglair and Galagoogoo variations reduce its base value of 4500 chocolate coins down to a mere 100.
Varianting pinatas does not affect romance, so you could romance an orange Whirlm with a purple Whirlm.
In addition to their color variants, some species are also able to evolve into a different species. For example, if you feed a buttercup to a (Classic) Sparrowmint, it becomes a Candary. Another well-known example is a Horstachio turning into a Zumbug after eating a blackberry and a daisy, as seen on the Horstachio of a Different Color TV episode.