
This is a design conversion from the Windows Phone 8.1 Settings Menu Concept:
It has been remastered to be from the first start screen from Pictures and Patterns:
As you can see the background accent translates to the background colour of both the pattern, and the settings app (Or any other app that utilises accents). The next colour choice is the accent colour, this colour follows the same traits as the already implemented accent colour in Windows Phone 8, however, it isn't the colour that is used for the live tiles when dealing with Patterns (With picture background or traditional black and white, or a solid colour, it is the live tile colour).
With patterns, the start screen live tile colour is automatically picked for readibility.
Sub-text colour will be picked automatically and won't be changeable. Although it will vary depending on the background accent and accent colour (It would be in the same colour range between both colours).
It could also be possible to have a live tile colour picked automatically based on the picture background, (These specific settings may all be defined further down in the theme settings menu).
This concept of colour choices has been a difficult concept for me to grasp, such as the ranges of accessible colours, based on the colours you have picked, Or the background image chosen. I have tried to explain how it works based on my findings in Windows 8.1 but if you have a better idea on how it might work post down below.
Here is an experimental version of the P&P design where the patterns are used like background accents in apps:

This is an experimental design, which I am not backing 100% because it can easily run into colour conflicts, unless it was handled better, If you have an idea on how it might work, post it in the comments.