[mark-r]

There's something that tends to go unrecognized, a function of the way our monitors work. Any color that is made of multiple primaries, such as magenta, cyan, or yellow, will naturally be brighter because more photons are emitted from the display. Not twice as bright, since our eye response is non-linear, but noticeably brighter.

[crazygringo]

Yup. This is precisely why the first image seems to have oscillating brightness, with clear sharp peaks at yellow and cyan. It's because it's not just changing color, it's literally twice as much light. It goes:

  Red - 1x
  Yellow - 2x
  Green - 1x
  Cyan - 2x
  Blue - 1x
  Magenta - 2x

(Of course magenta is not part of the spectrum.)

A very first step towards a better spectrum is just to maintain constant output brightness (accounting for gamma). There will still be perceptual differences in brightness, as we naturally perceive green as brighter than blue.

Obviously this gets taken into account by the time the author gets to the CIE color model. But there are a number of "intermediate" improvements like that, which you can make.

[ranger_danger]

Most monitor reviews I've seen measure levels for black, white AND grey.