I assume they meant the unintended rainbow-like color halos and vertical line artifacts plainly visible on a good crt, particularly on edges due to Woz's design where there is no direct pixel color really; rather, a "1" in a specific "phase" (odd or even column) of a pixel byte would tell the TV to output a certain color, like green or magenta. It simply walked the bits in memory as if they were chroma patterns, in rough approximation.

That said I'll read up on the linked article, I could me misremembering things.

Each “medium-res” pixel (140 across) on the screen could be seen to be three distinct colored pixels: red, blue and green. Purple was created by turning on the left two pixels, green the right most. Likewise red the leftmost and cyan the right two pixels. Only on a black and white display did you actually get 280 pixels without color artifacts.