The number of colours in a 24-bit colour image is 256³ or 16,777,216 colours. So how many colours are there in a 8 MP photo? Consider the following beautiful photograph:

In this image there are 515,562 unique colours. Here’s what is looks like as a 3D RGB histogram:

Most photographs will not contain 16 million colours (obviously if they have less than 16 MP, that’s a given). If you want to check out some images that do, try allrgb.com. Here is another image with more colours: 1,357,892 to be exact. In reality, very few real everyday photographs contain that amount of hue varieties.

Now as the average number of colours humans can perceive is only around a million, having 16 million colours in an image is likely overkill.