Getting Started with Nuke - Nuke 101: Professional Compositing and Visual Effects - page 22

Using the Viewer
Let's say you want to view the different color channels of the image you just brought in.
Here's how you would go about it.
1. While hovering the mouse pointer over the Viewer, press the R key to view the
red channel.
The channel display box on the far left is now labeled R. You can also change
which channel you are viewing by clicking in the channel display drop-down
menu itself ( FIGURE 1.19 ).
Figure 1.19. The channel display box shows R for the red channel.
2. Press R again to display the three color channels again.
You're now back to viewing all three color channels, and the channel display box
changes back to RGB ( FIGURE 1.20 ) .