With two bytes you can define 64,000 colors. This is the HiColor standard needed by some desktop schemes in Windows 95. With three bytes per pixel you can define 16.7 million colors. This is the TrueColor standard. I am told by experts that the human eye can detect well over a million colors.
Most color photographs cover that range.
Remember that we are not creating new colors but usually reproducing colors from existing pictures. To accomplish this, we need sufficient memory to store those pixels. These are standard increments. This is where you will now have to take out your trusty calculator. The memory needed on your card depends on the combination of resolution and colors that you desire.