CactiGuide.com

Melocactus is one of the few blue colored cacti. But for example why can’t the G. mihanovichii that comes in every mutant color be blue?

No cactus produces a blue pigment, Those that appear blue have a waxy "bloom" on their surface that scatters light, and short-wavelength blue light is scattered more than other colors. Gymnos don't produce a waxy coating.

I don't know how it works in cacti, but in reptiles green colour is result of combination of yellow colour contained in carotenoid cells and/or melanophore cells beneath the epidermis, and blue colour produced by the vertical organization of iridophores with thin platelets in the uppermost layer of the skin. So, if the yellow is not present (by mutation), skin appears blue to greyish-blue.

As Spence says any blue in cacti is caused by the scattering of light and not true blue pigmentation. Cacti cannot have true blue flowers either. The only apparently blue flower = Wittia (Psudorhipsalis) amazonica is caused by its textured surface scattering light as in Morpho butterflies wings and not actual blue pigmentation.

See:-

https://www.cactiguide.com/forum/viewto ... r%20plants.

Scroll down to see:-

https://asknature.org/strategy/wing-sca ... interfere/

If the demand on blue skin goes up the merchants will start spraying them that way, just whatch...