It is often banded with purple colours, which is presumably why the Romans regarded it so highly.
From the en.wikipedia.org
Inland, hippos roam free and the skies are filled with hundreds of bird species, including the spectacular southern-banded snake-eagle and purple-banded sunbird.
From the nzherald.co.nz
The copper rich mountains are banded in shades of pale green, brick red and a livid purple-grey, baked under the constant sun.