Colors 000 to 095 (That's 00 to 5F in hexidecimal) is known as the "sprite pallete." Most sprites (with a few exceptions) use the sprite pallete for their colors. By modifying the sprite pallete section in the pallete in the tileset you make, you can modify the colors to be whatever you want. This can be a very useful tool for eyecandy purposes because it makes the rabbits (as well as most sprites) blend in with the tileset, much in the way you want to make the rabbits appear monochrome.

For more information on tilesets, go here.