Canon/DIY Covens.

Are there any canon or DIY Covens with a Purple and White Color Scheme? I'm from the Space Marines side or things, and the lack of a concrete chapter equivalent is throwing me off.

Well, that's the thing I love about Xenos: there is no prescribed heraldy as with the marine chapters. Pretty much everything you come up with is DIY by lack of something official aside from maybe the kabals described in the painting section of the codex.

With covens you can do whatever you like, come up with a fancy sounding name for it and call it a day. You could even have your coven in the same colours as your other troops and say it is because your archon sponsors the coven, so their wear his colours.

I would note that even the most classic of all the Kabals, the Black Heart, has been shown to have at least two different color schemes in this edition alone by GW material only - so...it really is a catch as catch can as far as color schemes go.

It's the same sort of thing when people want to know why I don't build a 'fluffy' DE list, and my answer is - what's non-fluffy for them?

The apparent answer is 'anything that kills my opponent's army'.

I'd say invent or create away on your own. You may wish to go peruse our Aphotic Gallery to see if you get any inspiration though;
http://www.thedarkcity.net/f25-the-aphotic-gallery

Maybe that will help give you some settled ideas?