Tim Hunt

We can't debug this for you. You need to track this down. This is almost certainly bad data in your database.

To start with, while trying to diagnose this, change Debugging to Developer level to get more information about where the error is happening.

Just to note this was reported at MDL-82335, and I already made a comment there.

Which theme are you using? Looks like a non-stadard one, and that they have changes some, but not all the colours.

Learning to programme in other languages, not PHP, is probably a good thing. PHP is a horrible language (but you can write good code it in, if you have good development habits.) I am sure you will get used to the idiosyncrasies of PHP over time.

You can probably do this by overriding the right bits of the quiz renderer. https://docs.moodle.org/dev/Overriding_a_renderer