Channel Mixer Theory

Channel mixers work by converting the RGB channel values to grayscale (see RGB Color). The best way to explain it is to load the color chart into Photoshop and experiment with the channel mixer.
  1. Click here to download the color chart file and save to disk.
  2. Open the color chart file in Photoshop
  3. Create a new channel mixer layer by selecting
  4. set the Source Channel values to Red 100%, Green 0%, Blue 0%
  5. check Monochrome

Here is the resulting color chart.

Notice how the RED square has been converted to white and the BLUE and GREEN squares have been converted to black. Since we set the red channel to 100%, the values for the red channel will be converted to grayscale at full value. The value of the red channel for the RED square is 255. This is converted to grayscale 255, which is pure white. The value of the red channel for GREEN and BLUE is zero, which is converted to grayscale 0, which is pure black. Notice also the effect this conversion has had on the complementary colors. RED's complement, CYAN, is black, while the GREEN and BLUE's complements are white. Thus we come to the rules of channel mixing:

At 100% conversion the channel selected will turn:
1. the selected primary color white
2. the selected color's complement black
3. the other primary colors black
4. the other color's complement white

Here is the color chart converted at 0% red, 100% green, 0% blue.

Here's the color chart converted at 0% red, 0% green, 100% blue.

Of course the real power of channel mixing comes when you adjust the channels. This chart was converted at 50% red, 50% green, 0% blue.

The resulting grayscale values are:

RED 128
CYAN 128

You'll notice that adjusting a channel affects all colors with values for that channel. For example, adjusting the blue channel affects the BLUE, CYAN, and MAGENTA boxes. Is it possible to adjust the channel so that it affects only one color? Suppose you want to hold CYAN black and MAGENTA white while adjusting BLUE. How would you do it? The clue is in the RGB values for CYAN and MAGENTA. CYAN is (0, 255, 255) so it is affected by both the green and blue channels. MAGENTA is (255, 0, 255), so it is affected by both the red and blue channels. How do you turn CYAN black? The obvious way is to set both the green and blue channels to zero. But this will not hold CYAN black if you try to adjust the blue channel higher than zero. Another way is to set the green channel to -100%. This will set CYAN to black, and it will stay black as long as the blue channel is less than 100. You can now adjust the blue channel without changing CYAN.

To turn MAGENTA white, set the red channel to +100%. This will hold MAGENTA white while you adjust the blue channel.

It is possible to set the channel values from -200% to +200%.

Now that you know how the different channels convert the primary and complementary colors, you know which channels you have to manipulate to get the black and white look you want. For example, if you want to darken the sky (which is mostly blue and cyan), you'd want to boost the red channel and reduce the blue channel.

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