Out of these late experiments arose one of the most important capabilities of the Amiga, one absolutely key to its status as the world’s first multimedia PC. In the Amiga’s low-resolution modes of 320 X 200 and 320 X 400, Denise was normally capable of displaying up to 32 colors chosen from a palette of 4096. Miner now came up with a way of displaying any or all 4096 at once, using a technique he called “hold and modify” (HAM) whereby Denise could create the color of each pixel by modifying only the red, green, or blue component of the previous pixel.
http://en.wikipedia.org/wiki/Ham_%28chimpanzee%29