Nevertheless, almost all scientific research on colour vision is carried out with spectral colours
, whether it is humans who are asked to match a spectral light with a mixture of other spectral lights, or the unfortunate monkeys who are anaesthetized and paralysed so that a hole can be drilled in their eyes to measure the response of particular cells to spectral light flashes directed at tiny spots on their retina.
In Newton's colour circle, seven spectral colours
are specified on the circumference.
Other rarities are a "rainbow" calcite, which has the unique ability to separate and reflect light into its spectral colours
, and the world's largest faceted benitoite gem, a stone the colour of blue sapphire but with the fire and reflective properties of diamond.
Further investigation led them to the realisation that the spectral colours
of sunlight control the release of melatonin.
This is observed when viewing and tilting a SR hologram as parts of the light spectrum are observed as different spectral colours
that have lower intensities compared to the incident white light.