Melatonin production, sleep, and "cyan light"; how might this finding be possible?
Melatonin production is in response to photoreceptors in the eye, in part, the same photoreceptors in rods and cones that process and transmit information about the wavelength of light (eventually) to the visual cortex for vision. However, @uhoh was right to wonder whether there might be a separate receptor input for melatonin production.  

There is an additional opsin (photosensitive protein), found in the ganglion cells rather than the rods and cones, that is sufficient for responding to light and causing the production of melatonin by the pineal gland (mediated by the suprachiasmatic nucleus of the hypothalamus). Processing of light signals by the circadian system is quite separate from processing by the visual system. This is discussed briefly in Chapter 47 of Kandel's Principles of Neural Science, but you can read more about it in this review.

The pathways involved in receiving and processing light as an input to the circadian system were discovered relatively recently. Consider that the optic tract was discovered in the 3rd or 4th century BC, and the retinohypothalamictract was discovered in 1971.