Well, there's a (modest) learning curve involved in customizing color schemes and of course more complex tasks that are still in the domain of user's options.
Users can be fearful of "messing it up" if they change defaults. Making changes necessarily confers responsibility to follow instructions, learn how to alter settings and know the set of options that are appropriate to change and which are not.
That takes a pretty basic safety mechanism to address, require confirmation after the change. Windows has (had?) that, after 15 or 30 seconds or whatever from a change (like to resolution or something) it reverts back without confirmation. This makes changes of all sorts easy and cheap to perform. The worst case is you idle for 30 seconds waiting for it to go back to a legible form.
I think having the monochrome mode (which might be available at start time, and would also (temporarily) reset the font) would help with this and other problems (e.g. if one colour of the display is defective). This might be used for the UI to confirm the change but also when you start the computer that it can display such a message so that you can use that to recover from this and other problems (including screen resolution, colours, fonts, languages, and many more).
