What is the difference between masking a color in Channels & reducing the same color using Color Balance?

As Theresa said, Channels can also be used to make selections. To give an example of where this can be useful. If you have a model, standing against a background that you want to separate. You can look at each of the channels

and find that one has strong contrast between foreground and background (usually the red channel in this case). So you can use this channel to create a mask or matte. To do this I use Calculations to combine multiple instances of the channels together, using blending modes. This strengthens the contrast between FG and BG. sometimes I run Calculations more than once to acheive what I need.

You can then further perfect the mask by painting on it with Black and White, making sure the paintbrush is set to Overlay mode.

This is just one use of channels. Calculations is a great way to experiment with them as you can combine them

in different ways using blend modes. For example, I sometimes combine a channel with my main Image using blend modes to highlight or accentuate certain areas.

if you want to learn more about all of this I thoroughly reccommend the book, “the Art and Science of Digital Compositing” by Ron Brinkman. Its my bible :-)

Do you understand that selections are saved in the channels panel? And that you can quickly turn any channel into an active selection with the selection icon at the bottom of the panel, or by cmd or control clicking on a channel?

Hi

It depends on whether Preserve Luminosity is checked.

For example, without Preserve Luminosity checked reducing red by moving the slider to the left,  just reduces the levels of the red channel in a similar way to moving the gamma slider in a levels adjustment but with less range in the adjustment.

Start :

Reduce red

With Preserve Luminosity checked, reducing red reduces the levels (actually the gamma) in the red channel but also increases the levels (again in reality the gamma) in the blue and green channels

Edited to add the correct third screenshot

Dave