The following is from De Re Atari, Appendix D: "Television Artifacts":

   The ANTIC modes with which this can be accomplished are 2, 3, and 15.
   ANTIC mode 2 corresponds to BASIC mode 0, ANTIC mode 15 is BASIC mode 8,
   and ANTIC mode 3 has no corresponding BASIC mode.  Each of these modes has
   a pixel resolution of one half color clock by one scan line.  They are
   generally considered to have one color and two luminances.  With the use of
   artifacts, pixels of four different colors can be displayed on the screen
   in each of these modes. 

   The term TV artifacts refers to a spot or "pixel" on the screen that
   displays a different color than the one assigned to it. 

   A simple example of artifacts using the ATARI Computer is shown by entering
   the following lines: 

        GRAPHICS 8
        COLOR 1
        POKE 710,0
        PLOT 60,60
        PLOT 63,60

   These statements will plot two points on a black background; however each
   pixel will have a different color. 

   To understand the cause of these differing colors one must first understand
   that all the display information for the television display is contained in
   a modulated television signal.

For the rest of the De Re Atari article:
   http://www.atariarchives.org/dere/chaptD.php

Software can be designed to take advantage of artifacting to simulate more
colors than are available from the computer in a given graphics mode.  The
downside would be that, depending on the video display deviced used, the
actual colors seen are fairly unpredictable, or there may be no artifacting
effect at all.

A classic example of a game that utilizes color artifacting on the Atari is
the Br0derbund game, Choplifter.

More information about artifacting on the Atari 8-bit computers:

"Atari Artifacting" by Judson Pewther. Compute! #38, July 1983, p. 221:
   http://www.atarimagazines.com/compute/issue38/096_1_ATARI_ARTIFACTING.php
   or from Compute!'s Second Book of Atari Graphics:
   http://www.atariarchives.org/c2bag/page193.php

"GRAPHICS 8 In Four Colors Using Artifacts" by David Diamond. Compute!'s First
   Book of Atari Graphics:
   http://www.atariarchives.org/c1bag/page203.php

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