To expand, here is how I can imagine this working with the default themes:
For a given songs art, we choose a "light" colour (similar to what Retro Music Player chooses for text) and a "dark" colour (similar to what Retro Music Player chooses for the background.). If the user is using the dark theme, we use the dark colour as the background, and the light colour for text, icons, etc. If the user is using the light theme, swap them. This allows for day/night flipping alongside this option.
For 3rd party skins, we could just provide these dark and light colours as variables and let the developer use them as they wish.
When the song changes the colours smoothly fade into the next song's colours. No matter where you are in the UI.
If music pauses, fade back to the standard dark grey/white.
If the user chooses so, fade back to the standard dark grey/white whenever the user exits the "now playing" view.
I'm not sure how easy it is for colours to fade this way in the current implementation of Poweramp's theming engine, but I think it would be really sweet addition if implementated this way.