White desktop after Sequoia 15.4 upgrade

After updating to 15.4 my desktop was white, and the system refused to allow me to choose a custom colour. The 19 Apple colours would work, but they are all fairly disgusting, and I simply wanted to use my normal pale blue.

This seems to be a known issue on other forums and it is surprising that such a basic bug could have got into the operating system.

My work round was to choose the colour I wanted on the colour picker, screenshot the tiny spot of colour and use the add photo function, which has now restored my sanity.