Because other wavelengths were probably already taken. https://en.m.wikipedia.org/wiki/Purple_Earth_hypothesis https://en.m.wikipedia.org/wiki/Purple_Earth_hypo

Because other wavelengths were probably already taken. https://en.m.wikipedia.org/wiki/Purple_Earth_hypothesis https://en.m.wikipedia.org/wiki/Purple_Earth_hypothesis (Black is not a wavelength but rather the absence of visible light, e. g. it has all been absorbed)