Rat breeders have developed many different varieties and color variations. And, like dog breeders, rat fanciers have shows and beauty contests for their animals. Rats from breeders often come with pedigree papers, and you can get a look at the baby's parents (rat young are called "kittens") to see how easily they are handled and how healthy they look. Rats raised by most breeders have usually been handled and are used to humans. Consider getting two rats of the same sex to keep each other company.
For more information, please read the story Choosing a Rat.