Write some documentation about use of colours

:clipboard: **Description**

We've had a couple of PRs recently where contributors have wanted to contribute badges using branded colours in preference to the standard palette.

We have some pretty good docs we can point to when it comes to thing like conventions for badge URLs but I'd like to have some guidance written down about use of colours.

I need to have a think about exactly what to write, but the key point is basically: We use colour to convey information and ensure consistency, not to promote a project or brand. There may also be a few places where we need to pull some older services into line with this as part of writing the guidance.

This is related to shields values of being **semantic** and **non-promotional**

No file, test, or documentation entry point is named. Start by reviewing the existing badge URL convention guidance and the related semantic and non-promotional colour guidance, then identify older services whose colours conflict with the agreed policy. Done means the colour guidance is written and any identified inconsistencies are addressed.

Written by the indexing model from the issue text.