Hi @polymetric, and welcome!
Unfortunately we don’t have a region color picker, and Ctrl-Alt-LMB and Ctrl-Alt-Shift-LMB are not implemented either. See Color Picker: Ctrl+Alt+click should sample color from the input of the viewed node · Issue #96 · NatronGitHub/Natron · GitHub
You could do a trick with the ImageStatistics plugin to average the color over a rectangle, and either link or copy hte resulting color, but I admit that it’s not convenient.