Another, very "hardcore" solution:
- Color select and save selection as usual
- Enter quickmask
- Filter>Generic convolve
- Set all square surrounding the central one to 1, everything else (including the central square) to 0. Check the "normalise" option:
- this will replace all pixels in the selection mask by the sum of their surroundings, so isolated pixels disappear.
- then exit quickmask, Select>Grow, and intersect with save selection.
- Color select and save selection as usual
- Enter quickmask
- Filter>Generic convolve
- Set all square surrounding the central one to 1, everything else (including the central square) to 0. Check the "normalise" option:
- this will replace all pixels in the selection mask by the sum of their surroundings, so isolated pixels disappear.
- then exit quickmask, Select>Grow, and intersect with save selection.