Is there a way to drag to select multiple colors?

[rich2005]

Nothing as automatic as you want, that I know of.

Maybe use some helpers. Add two docks  Pointer and sample points.

The pointer shows the pixel info under the (selection) tool cursor.
Sample points can pre-identify points of similar colour to use. https://docs.gimp.org/2.8/en/gimp-sample...ialog.html