Color depth and byte size. For example, Netscape permits a choice from more than 16 million colors. Thus any pixel may have any one of these millions of colors. A bit is a computer's smallest unit of choice. That choice is either this or that , where this or that may be yes or no, true or false, on or off, zero or not zero, black or not black, etc.
Color resolution is the number of bits used to describe the color of one pixel.
For 256 colors this would be 8 bpp. One way to represent colors with three bits. 3 is blue or nothing. The size of the raw data that the image contains can now be calculated. From this we can see that even relatively small images take up a lot of computer storage space.