QUESTION: I’m doing a spoof of the Wizard of Oz, the scene in which the Wicked Witch sends off her flying monkeys. This will be done completely on (green/blue/magenta?) screen and the environment composited in post. The Witch's face is green, and the monkey's faces are blue and there is red on their suits. I need to figure out what color screen I should use.

Green Screen is the color of choice for many applications where a color-type key is needed, particularly when shooting with a digital camera.

For one, Digital cameras with bayer sensors have better resolution in green (there are twice as many green pixels as red or blue at the sensor level), and more importantly, the green pixels are of a much lower noise than the blue.

But to be successful, the green screen used should be of a sufficiently narrow bandwidth, evenly lit, and under lit such that that there are several stops difference between the green and the red/blue channel exposure.

On the other hand, the green makeup on the witch's face is probably not going to be so narrow-band. Provided there is enough image data in the red (or blue) channel, you may still be able to get a good clean key.

BUT DO TESTS.

I can not stress this enough. Make some tests with your intended camera, under intended lighting and against the specific green color intended for the screen, and a stand-in with the makeup on. Then load that footage into after effects, and examine the color of the screen using the eyedropper tool.

In a 32 bit linear colorspace, You should see values like this on the greenscreen portion of the image:

Note the very large differences in value between the red and blue channels. You need this kind of channel separation for the green screen.

After you do tests on the witch makeup, you may find that you need to make no real changes. Or you may find that you need to adjust it.

My suggestion is to make the witch makeup green but with a substantial red component. Then later in color correction you can do a secondary color correction on that specific color, and rotate the hue back toward green.

In the below example (not a real world test obviously), a small shift in the tone of the makeup was enough to keep the face out of the "green screen" and yet was very easy to selectively adjust the face color with nothing more than a 45 degree rotation of HUE, selectively applied only to the yellow/green color. 

And of course, you can always used Mocha to do a roto for the green portions of the witch, to hold them out from the keying software.

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