It looks like this is new to the 10.15.4 Beta but there is a button now under Preferences>Display for "Prefer HDR Mode" . After I installed 10.15.4 Beta this week, I noticed my external monitor colors had changed, and were super vivid. Unchecking the box for Prefer HDR Mode puts the displays back to the way they were before the beta, but if my Macbook Pro goes to sleep, or I unplug/replug into my dock, the vivid colors come back, but the HDR box is not checked. Checking and then unchecking it, puts the colors back to normal.
Curious if anyone else has noticed this.
2017 15" Macbook Pro
Same for me - it is a bug
Thank you so much for this. I wasn't sure why my colors were all messed up this morning.