One thing worth noting is that the HEX color representation is an abstraction of actual color. There are other color representations such as 48-bit color that can represent 281.5 trillion colors and CYMK that uses a 4 dimensional vector. All of these are simply abstractions of the visible light spectrum.
Also, the cones in eyes can generally distinguish 100 shades. Color blind people, typically only have 2 types of cones and thus can distinguish 10,000 colors (100x100). Most people are "trichomatic" meaning they have 3 types of cones and can thus distinguish 1,000,000 (100x100x100) colors. This is a far cry from the 16 million colors that can be represented with RGB. Although, some people (women exclusively) have an extra cone that allows them to see a wopping 100,000,000 colors. These people are considered "tetrachomatic".