It's only a couple of varieties of Aeonium that are purpleleaved and bright light will be necessary for the colour to be present. There are other varieties which are green.

its is a pigment that develops in the sunlight and 'masks' the normal green chlorophylls. There are lots of pigments but i dont know which one predominates in this plant.

Thinking about it they are probably one of the anthocyanin compounds. They relfect light from the blue/red ends of the spectrum. so your eye sees them as purple. I think they are a sub group of the flavenoids. But biochem is not my strong point.

Most plants will go paler if grown in insufficient daylight. It's called Chlorosis...Aeonium's version is to lose the dark red colouring.

You can grow both types though in their mediterranean home climate. The bright green will stay vivid and produce the fabulous yellow cone flowers in the winter.....the purple variety will take on a green tinge when growing after the rains then back to its dark maroon in the heat.