Hallo,
we can change BG or FG color when clicking on those small black and white rectangles.
If you click on the FG-color-rectangle, you can apply any color you
like (for example one color from twelve remembered by Gimp).
I have always one sample of 50%grey there in memory (introduced as html-color: 808080 ).
When needed - quick click on FG-color-rectangle, click on sample of 50%grey and then use the bucket-fill-tool to fill the layer or selected area of image.
I am a proud Linux and Gimp-user
