I found how to fix it, a file called theme.json appeared in December and had a very strange and limited set of colours in it. I have no idea why anyone would restrict the palette so severely but I presume the people behind the theme have a reason.
By replacing the entries with my own palette, I was able to restore the colours I used to have (plus add a few more).
I have now turned off auto updates on the theme.
I did try to add an image of the palette but seems it was necessary to first upload it somewhere and attach it as a URL and I didn’t have time to mess about with that. Not sure why a link to my website is needed as it wouldn’t have illustrated the issue – I would never have published a page that wasn’t correctly formatted.