Color Artifacts

The 320 color artifacts have two causes. The primary cause is alternating light and dark pixels which can create a strong 3.579545MHz signal either in phase with the colorburst (gold) or out of phase (blue). The second cause is each 320 pixel is only half the length of the colorburst waveform, therefore some TVs may have difficulty decoding the abrupt phase shifts. 

TV color (PAL or NTSC, composite or S-video) is encoded as the phase and amplitude of a colorburst frequency (3.579545MHz for NTSC). On the 7800 (and 2600) the amplitude is fixed (except for B/W when it's supressed) and only the phase changes.

Because in 320 modes the 7800 MARIA has a 2*colorburst pixel clock, the alternating on/off pixels of the tower in Tower Toppler creates a waveform at colorburst frequency. So rather than producing stripes it yeilds the solid color shown in the screenshots.  http://www.atari7800.org/screenshots/Tower_Topplerscreen.htm

The reason you can't see the light-dark luma pattern is because 

1. TV's try to filter out that frequency so you don't see the color carrier as a dot pattern and

2. The TV detects the high-rez luma pixels as a strong color signal with a high saturation value, so it adds brightness to the overall signal masking the dark pixels.

For color artifacts the color of the pixels isn't important, it's the repeating on/off pixels which cause the artifacts. The artifacts may allow colors to be shown which aren't in the 7800 palette, or display more colors than the given 320 mode is capable of.

Note: the artifacts do not appear with an S-Video mod.

If ColorKill is enabled then the 320 modes are displayed in black & white without any artifacting.