Set the username colour to the same as the rank colour (only on posts)
This Flarum extension changes the username colour on posts to the user's first rank. If they have no rank, the default colour will be used.
This only changes the colour on posts for now. This may be updated in the future to set the colour in all places.
To install the latest version compatible with your forum:
composer require mickmelon/flarum-ext-coloured-usernames