dt 3.1.0+1707~g1bd0d448b, Ubuntu 18.04
Using the left color picker by CTRL-clicking (area selection) the range shown in the diagram does not represent the color range selected in the image. The right endpoint seems to be correct, the left endpoint always stucks at red, the left end of the scale (see screenshot), independent of the selected area.
If one uses single left click (instead of CTRL-click) to select a single color, a range is shown in the diagram (instead of a line). The right end of the range seems to represent the selected color, the left end behaves like decribed above.
I did not observe this issue in version dt 3.1.0+1650, must have been inserted in the recent versions of git master.