The 'Tolerance' slider in the fill tool settings can be very helpful if the region you want to fill doesn't consist of a solid color, but includes variations on that color. When the fill tool is applied with a tolerance of 0, only instances of the original color are replaced by the new color. For example, if they color you are replacing is R = 50, G = 100, B = 150 it will replace only that color. If you have a tolerance of 6, it will replace all R = 47-53, G = 97-103 and B = 147-153, or mathematically [Component = Component +/- (Tolerance/2)]. This means not only the original color will be replaced, but colors slightly varying from that color as well.

Here is the original image.

Fill tool applied with a tolerance of 0. Only the original color gets filled.

Tolerance of 25. More of the wine color gets filled, but as the red color darkens deeper into the glass the colors are too different.

Tolerance of 50. The red colors darken quickly around the border of this fill and are still too different to change all the color

Tolerance of 75. Raising the tolerance more, we get slightly more fill.

Tolerance of 100. A bit more is filled here, but the tolerance is still too low to cover the darkest reds.

Tolerance of 125. Finally the tolerance covers a range where all the red color gets replaced.

Tolerance of 180. Now some of the shading on the glass falls into the color range.

Tolerance of 205. Additional grays and blacks are replaced.

Tolerance of 230. Still more grays are replaced.

Tolerance of 255. Finally all of the colors in the image are replaced.