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.

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.