|Wikipedia has more information about color depth.|
Color depth is a term describing the number of bits used to represent the color of a single pixel in a computer display. This concept is also known as bit depth or bits per pixel (bpp), particularly when specified along with the number of bits used. Greater color depth gives a broader range of distinct colors. Along with monitor resolution, it is one of the two factors determining how much video memory a Macintosh uses. Macs can be set to one of several color depths:
- 1-bit (2¹ = 2 colors) monochrome/black and white
- 4-bit (2⁴ = 16 colors)
- 8-bit (2⁸ = 256 colors)
- 16-bit (2¹⁶ = 65,536 colors), called "Thousands" in the control panel
- 24-bit (2²⁴ = 16,777,216 colors), called "Millions" in the control panel
Because memory is most conveniently accessed by programs in even multiples of a byte (8 bits), these are the only color modes supported. Non-Macintosh platforms sometimes are capable of "odd" depths, like 6- or 15-bit.
Indexed vs. direct color modes
At low depths, using an indexed color mode is common. In this mode, a palette of 2depth colors is allocated, with distinct colors assigned to each position in the palette. These colors need not be evenly distributed throughout the color space. An application essentially will call for "color 1", "color 2", etc., whatever that happens to be, having been predetermined to be the correct set. On the Mac, these palettes are allocated per-application, so that switching applications will cause the entire desktop to shift (sometimes jarringly) to a new set of colors that may make the desktop and background applications look strange if "color 1" becomes very different than the previous.
As the depth becomes higher, allocating this palette consumes too much memory, and a direct color mode becomes more common. In this mode, a certain number of bits are given to represent the red, green, and blue components of the color. The colors that can be displayed onscreen are whatever combinations of those three are possible. Since human vision is most sensitive in green light, "extra" bits are usually given to the green component so that more shades of green are possible.