16-bit color

16-bit color is a color depth that can allow for up to 65,536 unique grays or color values. However, in an RGB color space, the values can be assigned in a few different ways.

NeXT
In color systems from NeXT, the first 12 bits define three 4-bit channels. The resulting 16 red x 16 green x 16 blue values create 4,096 possible combinations of unique color values. The remaining 4 bits can be assigned to an alpha channel to define 16 levels of transparency.



Apple
In supported color systems from Apple, the first 15 bits define three 5-bit channels. The resulting 32 red x 32 green x 32 blue values create 32,768 possible combinations of unique color values. The remaining 1 bit can be used to define a single level of transparency. This is referred to as "thousands of colors" on Macintosh systems and "high color" on PCs.

Other systems
In some 16-bit systems, the last remaining bit is not used for transparency and instead added to the green channel to double the number of possible values because the human eye has greater sensitivity to this color. The resulting 32 red x 64 green x 32 blue values create 65,536 possible combinations of unique color values. This is referred to as "direct color" on PCs with XGA support.