The BBC News article Cyan colour hidden ingredient in sleep describes research that suggests melatonin levels as measured in saliva could be affected by the presence or absence of cyan color in a displayed screen, even when the color balance was adjusted so that the difference was not visibly perceptible.
The colour cyan - between green and blue - is a hidden factor in encouraging or preventing sleep, according to biologists.
University of Manchester researchers say higher levels of cyan keep people awake, while reducing cyan is associated with helping sleep.
The impact was felt even if colour changes were not visible to the eye. (emphasis added)
The researchers want to produce devices for computer screens and phones that could increase or decrease cyan levels.
Is cyan going to keep you awake?
I am guessing that this even possible because color perception comes through the sensitivity of three (or four?) color channels and so two different spectra can appear to be the same color. An example might be that a green wavelength might appear to be the same color as the mixture of a blue and a yellow wavelength if properly adjusted.
But if the two stimuli are adjusted to have the same or similar mix of signals in the individual's color receptors, how might the melatonin production still be affected?
edit: According to Wikipedia, monochromatic light would have a wavelength in the range of 490 to 520 nm, and if you wanted to create a visual impression of cyan, you'd use a mixture of Green and Blue channels:
Wavelength 490–520 nm
Hex triplet #00FFFF
sRGBB (r, g, b) (0, 255, 255)