RGB

RGB refers to an additive color model made up of Red, Green, and Blue light. Each channel of component color combines with the other two to to form a range of unique colors. In an RGBA color space, an additional Alpha channel can be assigned to define opacity / transparency.

12/15/16-bit RGB color
In 16-bit color mode, the RGB values can be defined in a few different ways:

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.

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.

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.

24/32-bit RGB color
In 24-bit color mode, the RGB values are made up of three 8-bit channels. The resulting 256 red x 256 green x 256 blue values create 16,777,216 possible combinations of unique color values. This is referred to as "millions of colors" on Macintosh systems and "true color" on PCs.

In modern systems, 32-bit color graphics allow for 8 additional bits to be assigned to an alpha channel, which can be used to define 256 levels of transparency.

Deep color
Newer systems support extended ranges of color from 30 to 64 bits, referred to as "deep color". The first system from Apple to include built-in 30-bit support are the 2014 retina iMacs running OS X 10.11, which can display 10 bits per channel for up to 1,073,741,824 unique colors. Another example on macOS systems using the 64-bit version of QuickTime X is ProRes. The ProRes 4444 codecs support three 12-bit channels that can create 68,719,476,736 possible combinations of unique RGB color values, along with a 16-bit lossless alpha channel.