The Hue Match Mode chooses pixels based on their position in the palate. Hue is the actual primary color, i.e. Red, Green and Blue. Lets take a look at an example. In the above example the selection is rectangular. This is due to the way the pixels are aligned.
Lets take a look at examples using the red circle and the surrounding blue pixels.
In the above examples, the Hue Match Mode was used and the tolerance was set to 10. Notice how almost the entire red circle was selected when a pixel in the red circle was selected. When a pixel in the blue area was selected, most of the blue area was selected. By increasing the tolerance, even more of the blue pixels would be selected including some of the greenish pixels. I applied a faded edge mask.