Does anyone know why the pottermore background is purple?

The background is a dark and rich shade of purple..
that shade gives out a very magical, mysterious and mystical look. These are the perfect words to describe pottermore because first of all its magical, and its still a slight mystery of what exactly we'll be able to do in pottermore and what other features are there.. we know the basics but not exactly what it is..
I hope this answers your question! ^^

Purple is the color of mystery, spirituality and sacredness. Well, according to some traditions.

Perhaps, they did not wish to be partial to any house by choosing either green, blue, red or yellow, if you see what I mean.

Purple is a magical color.

^It reminds you of magic...maybe...