Depends on what you do, but 24 bit is SVGA true colour

Color bit depth refers to the number of bits used to describe the color of a single pixel. The bit depth determines the number of colors that can be displayed at one time. Here you can see the number of colors different bit depths can produce:
Bit-Depth 1
Colors: 2 (monochrome)
Bit-Depth 2
Colors: 4 (CGA)
Bit-Depth 4
Colors: 16 (EGA)
Bit-Depth 8
Colors: 256 (VGA)
Bit-Depth 16
Colors: 65,536 (High Color, XGA)
Bit-Depth 24
Colors: 16,777,216 (True Color, SVGA)
Bit-Depth 32
Colors: 16,777,216 (True Color + Alpha Channel)
Notice that the last entry is for 32 bits. This is a special graphics mode used by digital video, animation and video games to achieve certain effects. Essentially, 24 bits are used for color and the other eight bits are used as a separate layer for representing levels of translucency in an object or image. Nearly every monitor sold today can handle 24-bit color using a standard VGA connector.