Improve text color for certain label colors

[Andre601]

Certain colours make the text on the right harder to read. This is the case for green where the white font is a lot more difficult to see than with other colours.

Shields.io should have a system that would adjust the font colour if the colour used would be too bright for the normal white colour.
Alternatively (Or in addition to it) could shields.io offer a query option to override the font color for the left and right side of the badge.

Here's an example displaying the issue with the bright color:

[Andre601]

Well, it apparently does already change font colour (i.e. for colour 9cf), tho it seems that green (And a few others) don't count towards this.

Colors I noticed to have hard to read text:

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[calebcartwright]

Relevant backstory and reasoning in #5467 and #5467 (comment) and #5497