MacOS 15.4 wallpaper color picker broken

Correct. It's a bug in 15.4. You can send a bug report from the feedback page.

Feedback - macOS - Apple

I have experienced this behavior after the update as well and have reported it to feedback.

I had three white screens after updating this morning.

Here is my solution: I especially like the old Apple color that was called Kelp. So I created a Photoshop file (could use Preview just as easily) that is just a kelp-color solid (Hex# 5DAB95 or R=93; G=171; G=149 is pretty close); place that image in Photos and chose it as a wallpaper.

In Apple feedback they never have the latest version listed. This unfortunately is always prevalent, and it is Apple not up to speed. However, the bug re the solid colour has been reported by lots of people and yet nothing has been done to fix it. I would hope that they fix it pronto, but of course Apple never seem in a hurry to fix anything, and often do not bother and one has to wait for yet another update to see if it has been rectified.

I, like another poster also had a white screen after the update, and it's beyond me how Apple are able to mess up customers settings when offering an updated version?

And yes I have reported this through Apple agents on the phone as well as on the Feedback form.

I have the same issue.

Also they release an update and when you want to report a bug the newest version is not even available from their OS drop down selection.