I just want to follow up that I encountered this behaviour in UE5 and it seems to be specifically when using the color picker. What I realized was happening was I was color picking from a different UI Image, which was not just picking but also selecting the other UI Image. The Image I was trying to change the color of appeared to have changed, and the color selection window stayed open (I even Clicked the Ok as described above). The Image I was trying to change appeared to be updated when I navigated back to it but when compiling it reverted back to the original (incorrect) color.
So basically I have two UI elements UIA and UIB. I have UIB selected, color pick from UIA. UIA is now selected, UIB appears to have its color updated, however when I compile, the UIB color reverts to what it was before. The tooltip with the Ok button is still open, but even when I press Ok it doesn’t save the changes properly. Weird and obscure issue but hopefully this helps someone else.
I’ll also just add that I was able to get this to work by color picking, copying the Linear Hex code for the color, reopening the color selection window and pasting the value in manually.