If a light-dark alternation (let's call it day-night) is displayed at a frequency of several cycles per second, one sees a flickering gray. Now if a line drawing is briefly exposed during the day, the drawing's black ink seems tinged with color, and the color varies according to the timing of its exposure during the day, as suggested in this sketch:

C. E. Benham in 1894 made this into a popular toy in England, a spinning disk decorated with annular patterns of black and white called "Benham's top" (Figure b):

The mechanisms of this perception remain only partly disclosed, but they seem to involve phase-sensitive lateral interactions among neurons, both in the retina and in the brain. You might consult the 1995 review article by von Campenhausen and Schramme ("100 Years of Benham's Top in Color Science",  Perception 24, 695-717) but don't expect definitive explanations.

 

Figure b: When this disk is rotated clockwise at 300 rpm, the innermost rings appear red and the outermost appears blue (or vice versa if anticlockwise). The middle rings are greenish. Click image to enlarge

Despite ignorance of its physiological origins, television engineers in the 1950s managed to parlay Benham's Top into a scheme for broadcasting color movies over black-and-white transmitters and black-and-white home receivers. The device is called the Butterfield color encoder. Three consecutive color filters during the "day" are arranged on a rapidly spinning wheel to alternate with a "night" of three consecutive opaque segments in front of the television camera. The red-blocking filter immediately follows the night: anything red looks black. Then comes a green blocking filter, then a blue-blocking filter. Black lines from the early red-blocking filter are seen early in the day and so they appear red, and those seen in the middle appear green, and those seen at the end appear blue. So this phased presentation of the three complementary color images reconstructs a colored picture on the black-and-white television screen. Its first public demonstration was a soft-drink advertisement over station KNXT in Los Angeles. Nothing was said about color, to the consternation of thousands of viewers who seemed suddenly to be hallucinating. Unfortunately the perceived color proved to vary from individual to individual and it depends unpredictably on the colors of its surrounding background. Moreover the effect works best at five to ten cycles per second: the annoying flicker of the color picture was so distasteful to television audiences that they preferred to buy the more costly RCA tri-phosphor system or to forgo the color and stick to black-and-white transmissions.

I have a demonstration on VHS tape (a carpet soap advertisement)  if you want to borrow it to watch on a black-and-white monitor.

Reminder to the reader, and especially to the author:  This insert seemed to me too fascinating to leave out, but it is exactly the sort of thing I am trying to not produce. It is didactic, declarative, and instructional: the sort of thing professors and preachers do. You can get info elsewhere. My special aim in learning to write a column here is to create a different literary style in which personal experience of questioning and probing exclusively holds the center of attention. I will try again next time.