As c.pfaffenbichler stated, you are not really working with channels when using the hue/saturation adjustment layer. You can select different colors and control the hue saturation of them independently, as you mentioned by selecting red, green, etc. You can also fine tune what colors are selected by adjusting the sliders for the range at the bottom of the dialog box. You might just want to boost the saturation of all the colors, hence you would use the master selection, and then fine tune that adjustment by selecting other colors.
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