Channels
Main Menu: Image > Adjust > Channels
The Adjust Channels window allows modifying selectively the brightness in each image channel.
Each tab in the Channel window is intended to adjust the image pixels colors, by using different channels arrangements.
The image may be divided into different channels:
RGB: Red - Green Blue
HSL: Hue - Saturation - Luminosity
If the entire image is not visible in the image window,
you can use the navigation buttons 
to bring another area
of the image into view or to zoom in or out the image.
Click Restore to
undo any changes applied to the image.
Only if Auto Apply is selected
or if you selected Apply once,
changes will be applied to the image, otherwise the will be only previewed.
To apply the changes to the image either click Apply, select Auto Apply or click OK to apply the changes and close the window.
Use the R buttons, close to each channel slider, to reset that channel to 0.
See also Color Channels