Use different and uniform coloring for badge icons

Badges can end up with logos in different colors which can be quite a mess, or just less readable because of bad contrasts for example.

See https://github.com/badges/shields/pull/3294 for some more information.

Start by reading the linked pull request #3294 for the background and proposed direction. Identify the badge icon coloring cases covered there, then define a consistent, readable coloring approach and verify that the affected badge logos no longer produce poor contrast or mismatched results.

Written by the indexing model from the issue text.