Bit Depth

Bit Depth (bits per pixel) Number of colours or tones
1 2
2 4
3 8
4 16
6 64
8 256
16 65,536
24 (3 x 8 ) 16,777,216
32 4,294,967,296

Colour Lookup Tables (LUTs) give you a choice of more colours

One stores only the index to a set of color chosen amongst a larger set

E.g. : 16 bits pictures, chosing amongst millions of color instead of thousand
300x200 image cold anyway need up to 60000 colours !