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 !