The color depth indicates how many different color levels are available for each individual pixel of a graphic. Since the “fineness” of the gradations depends on how much memory is used per pixel, the color depth is specified in bits.
With 8 bits, for example, 256 color shades can be distinguished for one color channel. A color is created by mixing several color channels of a color space. For computer graphics, the RGB color space is usually used, in which colors are composed by additive mixing of the three primary colors red, green and blue. Even most common computer monitors can only distinguish 8 bits per channel.
One speaks of a true-color representation if the current color depth has at least 24 bits, i.e. 8 bits per color channel (red, green, blue). A color depth of 24 bits corresponds to approx. 16 million colors; this means that practically every conceivable color can be reproduced true to life.