When I mentioned being able to pick a color on my own, I meant on a per album basis. Sort of like a setting in the same tab as the one you use to change album artwork for a single album. Besides implementing the color picking UI, it could be as simple as a user choosing a color and a HEX value being saved in the settings or in the album folder.
I know this is sort of a niche feature and the app wasn't designed to do this but that's why I'm posting in the feature /request/ forum. obviously aesthetics aren't the most important thing to focus on but Poweramp has really grown into itself with V3 and I think this simple feature would compliment the app's visual design well.
However, being that it seems like only two people here are interested in this feature, syncing the background colour with the notification widget color picker might be the easiest option to implement. Either way, I just thought i'd throw the idea out there because I think a uniform solid color looks way better in the player than having a slightly faded version of the art as a background.