TL;DR: Disable True Tone
So the situation is that you have a macOS laptop (MBA, MBP) and you want to have 2 screens (the laptop screen as well as an external screen). If you just use the external screen with the laptop closed, the brightness and colors look a particular way (good). When you open the laptop lid and the configuration switches to extended mode, the colors on the external monitor become dulled and muted.
The reason for this is the "True Tone" feature. You need to disable it, by toggleing the "True Tone" feature to off in the Display Settings for the built-in screen (not the external one). When you do this, the external/extended monitor will go back to looking as it did before you opened the laptop.