Dithering

For the pixel art illustration, I use a very limited pallet made up of three 'colors'; light gray, dark gray, and black.
By using a limited pallet, I am forced to make use of dithering.

Dithering is used to create the illusion of color depth in images with a limited amount of colors.
By using the available colors and mixing them, in-between colors are mimicked. Depending on the lack of available colors, dithering becomes visible as grain.
Dithering takes advantage of the human eye's tendency to mix colors in close proximity to one another.

If you want to save a picture of a photograph as a GIF file, you would probably need to use dithering.
A picture of a photograph has a lot of different colors and gradations, whereas the GIF file format only supports a limited amount of colors, 256 at most. The computer will use the 256 most frequent colors; the remaining colors will be approximated by combining the same 256 colors.

the complete page is available in the commercial version of this tutorial.