32-bit color
From The Glossary of Digital Photography
Similar to 24-bit color, except that an extra channel of 8 bits, called the alpha channel, is added. As in 24-bit color, these other bits are broken down into 3 bytes; each byte has 8 bits and represents one of the three primary colors: red, green, and blue. On PC computers this is still called true color and on Macintosh computers it is called millions of colors, as in 24-bit color. The actual number of colors available with 32 bits is calculated by looking at each color, which is represented by 1 byte; each byte has 8 bits, which means 28, or 256, colors for each byte or color. Since there are three colors, there are 16,777,216 (256 x 256 x 256 = 16,777,216) colors available for each pixel, the same as 24-bit color. The extra byte of 8 bits is represented by the alpha channel, which can be used for masking information, effects, or when compositing several images. Sometimes the alpha channel is left empty and used for padding a 24-bit color image up to a 32-bit color image, since modern computers often use 32 bits for their processing. See also Alpha channel
