saulgoode wrote:
In either case, the "full-screen eyedropper" is accessed with the button that I circled in red in the above screenshot. After clicking on that button, GIMP will wait for you to click somewhere on the screen, and then your foreground color will be set to the color under the cursor. After picking your color, the cursor returns to its normal mode.
Thanks for the tip, saulgoode. I had no idea that Gimp's color picker worked on areas outside of gimp, like a webpage. I just tried it on a website and worked perfectly.

I have kept the 3 window mode of gimp, it might be difficult for someone using single-window mode.