Disable low contrast / same shade as background colour options for usernames
The problem is that as it is right now usernames become unreadable due to a feature only available for Turbo subscribers.
This is unrelated to Chat Appearance: readable colours as the Chat Appearance: readable colours changes the tone of EVERY chatter, and by the few users that use unreadable colors the entire twitch community should not be restricted to the brownie, muddy colours that chat Appearance: readable colours
My suggestion is to restrict the color pallet available to users to colors that conform to a minimum of 4.5:1 to match internet accessible standards. Give users who are paying Turbo the option of High Contrast a a color so that their name will look black on a white background and white on a black background if they want a monotone chat appearance.
It will also offer additional room for improvement to the Chat Appearance: readable colours toggle as it will offer an even higher contrast ratio for users that really need it and not for users that are just trying to deal with super low contrast names,