How Subtractive Color Mixing Works

I have a fantastic tool that I use to teach subtractive color mixing in my color lectures and workshops: it's a deconstructed, full-color glass gobo that allows me to display its individual cyan, magenta and yellow layers.

The Cyan, Magenta and Yellow Layers of a full-color Aztec Mask gobo

Just like CMY color-mixing automated fixtures, a full color glass gobo creates all of the final colors you see using the “subtractive primaries” cyan, magenta, and yellow.

Breaking this down simply:

The cyan layer is subtracting or filtering out the red end of the spectrum (long wavelengths) and allows the green and blue (middle and short) wavelengths to pass.

The magenta layer is subtracting or filtering out the green middle of the spectrum and allows the blue and red (long and short) wavelengths to pass.

The yellow layer is subtracting or filtering out the blue end of the spectrum (short wavelengths) and allows the red and green (long and middle) wavelengths to pass.

Going to the next step, I show how stacking the cyan and magenta layers “subtractively” mixes to blue, combining the magenta and yellow layers results in red, and stacking the yellow and cyan layers creates green. Gradients of the subtractive primaries are used to achieve the varying hues and saturations needed in the gobo.

Pairs of the Aztec Mask gobo layers mixing to blue, red and green

If all three primaries are at their full saturation, the result is almost black. This is because, in subtractive mixing, you are filtering out wavelengths rather than adding them. Thus, layering all three subtractive primaries will filter/remove all of the light.

CMY filters are not perfectly tuned to every light source, though. Hence some light still gets through. Automated fixtures have a douser to take care of that last little bit. A full color gobo uses an opaque black layer. Stack the cyan, magenta, yellow and black layers of the gobo and you get THIS:

You might notice some slight color aberrations in the gobo above – that’s because it was me that stacked the layers by hand, not any of our world-class gobo makers. That said, now that you know how the gobo is made, notice how little of the original cyan, magenta, or yellow are in the final image! Instead there are reds, greens, vivid ambers that almost feel metallic, and several other complex colors – all created by the power of subtractive color mixing.

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