Keying - Nuke 101: Professional Compositing and Visual Effects - page 318

When using Keylight, the first thing to do is connect the background if you have
it.
3. Connect Read1 to Keylight1's Bg input.
Now you can begin keying by choosing the screen's main green pigment.
4. Click the sample color from the Viewer button for the Screen Colour property to
activate it ( FIGURE 7.26 ) .
Figure 7.26. Turning on the Screen Colour property's sample color from the
Viewer button.
Note
Remember, when capturing colors from the screen, holding down
Ctrl/Cmd activates the actual selection. Adding Alt/Option chooses
colors from the input of the node, rather than the output. And adding
Shift lets you pick several pixels and then average between them.
5. Now, while holding down Ctrl-Alt-Shift/Cmd-Option-Shift, click and drag on-
screen to find the best screen color. Look at FIGURE 7.27 —the red line shows