Here's an app for people enjoying Start backgrounds on 8.1:
Accent Builder
Accent Builder is a twist on the idea of creating solid colour backgrounds for your Start screen, going further by allowing you to set multiple columns of colour to align with your columns of tiles. This can help you categorise your Start screen vertically using subtle visual clues, or can even simply be used for artistic effect.
You can use up to 2 colours at once for free, and the paid version (99? US via IAP) unlocks up to 6 colours at a time (one for each available column of small tiles). Your configuration syncs across all Windows Phone 8.1 devices on your account, and there is an update coming shortly that will allow you to save multiple configurations for future use.
Check it out below; I appreciate your support!
Accent Builder | Windows Phone Apps+Games Store (may still be rolling out to some regions; try it on your phone and it should work).