The illustration shows a photograph made in 1980. It was a sextuple exposure on colour slide film. The subject, an antique watch, was photographed six times on the same frame. Each of the six exposures was made using one specific filter. Red, green and blue filters were each used on two of the six exposures.
The overlapping regions in this photograph clearly demonstrate how the additive primaries combine:
green + blue = cyan
red + blue = magenta
red + green = yellow
red + green + blue = white