Apple - External display color changes when MacBook lid is closed

If you update to macOS 10.15.4 it may now detect that you have a HDR screen, and makes your external screen colors look weird.

To fix this:

• Go to Display settings
• Uncheck "High Dynamic Range" for your external display
• Everything should be back to normal.

I had the same problem. My external monitor doesn't support HDR, so PaulVO's method didn't work for me.

As Nimesh suggested, I unchecked the "True Tone" for my Mac retina display. I immediately saw my Mac display had the same blue tint. Then I was able to calibrate both the color of my Retina display and external monitor. Somewhere between D50 and D65 works best as qrius suggested.

Thank you all for the inputs!