What are 24 bit, 32 bit and 16 million colors?

Red          8 bits per pixel = 256 shades of red
Green      8 bits per pixel = 256 shades of green
Blue         8 bits per pixel = 256 shades of blue

Alpha       8 bits per pixel = 256 shades of transparency for compositing

8 bits per channel x 3 channels RGB = 24 bit total per pixel.
8 bits per channel x 4 channels RGBA = 32 bit total per pixel.

256 red x 256 green x 256 blue = 16,777,216 colors available per pixel

Human Vision
A person can distinguish between 150 different shades of a color.
150 red x 150 green x 150 blue = 3,375,000 visible shades, Is this true?