These colorful cacti are actually two plants that have been grafted together. The reason for this is that the colorful tops have no chlorophyll (remember that word from 5th-grade science class?) that the plant needs to photosynthesize. They are grafted to another cacti that can provide enough nutrients for the both of them.
 
Note: Sizes are indicative of pot size, not plant size. For example: 4" pot means that the plant will come in a 4" tall by 4" wide plastic nursery pot.