Seeing things? Purple clouds

It hasn't photographed as well as I'd hoped but for much of the morning on Sunday the clouds above Skiddaw looked a definite shade of violet, perhaps lilac. It was heading towards the middle of the day so we're not talking a sunset effect, which is all I've got when I've tried searching on the Internet. The Sun was behind me. This effect was not visible on the clouds on the same side as the Sun. So more observations will be needed. My companions agreed that it was true, but is it a physiological effect? Is it to do with the snow cover on the mountains? Is it to do with scattered light coming down through the clouds or is it light scattered back from the snow fields?