The basic rule is that the speed of a card is determined by the controller, the memory speed, the software drivers, and to a much lesser degree the board design.
Color depth is determined by the amount of memory.
The amount of memory is not intended to affect speed. This is not a valid color depth for 3D games. There are 20 base colors. The higher the color depth means more shades of the base 20 colors. This is the most common color depth for 3D games.