Bits per pixel

Color depth or colour depth (see spelling differences), also known as bit depth, is either the number of bits used to indicate the color of a single pixel, in a bitmapped image or video frame buffer, or the number of bits used for each color component of a single pixel. For consumer video standards, such as High Efficiency Video Coding (H.265), the bit depth specifies the number of bits used for each color component. When referring to a pixel the concept can be defined as bits per pixel (bpp), which specifies the number of bits used. When referring to a color component the concept can be defined as bits per component, bits per channel, bits per color (all three abbreviated bpc), and also bits per pixel component, bits per color channel or bits per sample (bps). Color depth is only one aspect of color representation, expressing how finely levels of color can be expressed (a.k.a. color precision); the other aspect is how broad a range of colors can be expressed (the gamut). The definition of both color precision and gamut is accomplished with a color encoding specification which assigns a digital code value to a location in a color space.

32 bit.png 16,777,216 colors
98 KB

8 bit.png 256 colors
37 KB (-62%)

4 bit.png 16 colors
13 KB (-87%)

2 bit.png 4 colors
6 KB (-94%)

...
Wikipedia