It's something to keep in mind when designing sprites. When designing characters or creatures or anything else that will be implemented as a moving sprite object, remember that "sprites" are divided into tiles. A 16x16 pixel sprite has four tiles, a 32x32 pixel sprite (big for the NES) has sixteen tiles, and so on. Each of the tiles within a sprite can have one of two separate palettes assigned, each palette consisting of four colors (one of which is reserved as the alpha/transparency), but the palettes must share a common alpha color, but that leaves you with six unique color values available for a single sprite. You're constrained to one palette, or rather, three visible colors, per tile, but carefully planning the design of sprite, i.e. paying attention to the tile divisions, can let you really push the color composition of a sprite on the NES without any real additional effort like parented objects and some other advanced voodoo (like Mega Man's face).