This may be old news to those who are up on the vision neuroscience literature, but I found it interesting and surprising so I thought I would share. It appears that within the human population, individuals vary in the number of red-cone genes (from 1 to 4) and green-cone genes (from 1 to 7!) they carry. Neitz & Neitz, (1995) found all of these variants in a sample of only 27 men. Further, these different genes are expressed as different red and green opsins. So, there appear to be plenty of human tetrachromats, pentachromats, hexachromats, etc. running around.

Neitz, M., & Neitz, J. (1995) Numbers and ratios of Visual Pigment Genes for Normal Red-Green Color Vision. Science, 267, 1013-1016.