Color modes determine the number of colors that can be displayed in an image, and affect the file size of an image. Increasing the number of colors in an image also increases the file size.
1 Bit images are made up of one bit of color (black or white) per pixel, and requires the least amount of disk space.
Grayscale images are made up of 8 bits of information per pixel and use 256 shades of grey to simulate gradations in tone..
Indexed color images are single-channel images (8 bits per pixel) that use a color lookup table containing 256 colors.
RGB color images use three colors to reproduce up to 16.7 million colors on-screen. RGB images contain 24 (8 x 3) bits per pixel.