Correcting skin tones with curves
Photographs with people as the main subject show bad colour or colour casts very obviously. Because we see other people everyday we all have an instinctive knowledge of the shades of skin colour. For example everyone knows that people are never a shade of green, unless they are very sick. So if your photograph has people with bad skin tones it stands out a mile.
Fortunately we can use curves to correct skin tones too. In my previous post about using curves in Photoshop to colour correct images we went over the basics and then corrected a colour photo by setting the black point, white point and middle grey. If you are new to using curves or Photoshop then please read my post on the histogram and using curves.
Even though humans come in a wide range of shades we can fix skin tones in a photo using an ideal colour for each type of skin.
We’ll work in RGB but view the colour mixture in CMYK.
Caucasian Skin – Both Yellow (Y) and Magenta (M) should be about the same, slightly more Yellow is fine – it makes the skin look warmer, but the difference should be more than about 10-15. Cyan (C) should be about a quarter to a fifth of the Magenta value. There should be no Black (K).
Asian/Hispanic Skin – Yellow should be greater than Magenta by about 15. Cyan should be a quarter to a third of the Magenta. There should be no Black.
African Skin – Dark skin can be quite varied. It ranges from light brown to very dark brown and can veer towards maroon / purple tones. Yellow and Magenta can be the same or there can be more Yellow than Magenta depending on the shade. There should be a lot of Cyan anywhere from a third to half of the Magenta value. It can also contain Black up to one half of the Cyan value.
Putting it into practice
Open your image that needs fixing. Select the eyedropper tool and make sure it is on the Colour Sample Tool, the one with the cross hair next to it.
When using the eye dropper to find the colour mixture of the skin open the curve dialog box and aim for somewhere around the top quarter of the curve. Hold down the Command key on Mac or Control on PC to make a anchor point on the curve. Don’t take the value from an place where you might get a bad reading such as areas with make up or an area that is obviously different such as a blushing cheek. You might want to take a couple of samples.
Make sure the info panel is open. In the info panel you’ll see reading for sample #1. It will probably be in RGB to start with. Just click on the eye dropper icon and change the reading to CMYK. Check the CMYK breakdown and compare it to the ideal colour mixture for the skin tone of the person as listed above. We can see our CMYK mixture is quite away from the ideal. There is too much Cyan (C) and too much Black (K).
By looking at the photo and the fact there is so much black I think it can do with lightening overall. So we’ll open curves and just lift up all the channels in the middle tones.
While we are taking the reading as CMYK the image is still RGB. So we need to understand the way curves work in RGB. To help you open up the Colour Balance dialog box. On the right are the 3 channels of an RGB image. Red, Green and Blue. On the left are Cyan, Magenta and Yellow.
These sliders work in a similar way to the RGB channel curves. For example if we remove Red the overall colour of the image will move towards Cyan.
If our image is too Blue then open the Blue channels curve and move the control point you made earlier down a small amount. Take notice of the CMYK mixture in the information panel. As you move the control point down, the Yellow percentage should rise.
We need to decrease the amount of Black and Cyan. Click on where your sample cross hair is while holding down Command + Shift on Mac or Control + Shift on the PC. This will put a control point on the curve of every channel. We need to get rid of some Cyan. So go to the red channel and lift up the control point. This puts in more red and decreases the Cyan.
The Cyan has gone down from 30% to 12% but the Magenta has gone up as a result. Our people have gone very red but that’s ok we’ll fix that with the other channels.
Open the Green channel and move the control point up slightly. This will decrease the amount of Magenta.
The Magenta has gone down to 41% but the Yellow has gone up because Green contains Yellow.
Open the Blue channel and lift up the control point to add Blue. Blue is the opposite of Yellow, remember the Colour Balance box?
The Yellow has come down to the same as Magenta on the sample point #1 and it is a little higher than Magenta on sample point #2 so that is fine.
So our people have a more pleasing skin tone than the dark muddy colour they started with.
Remember the colour mixtures are a guide so you don’t have to match them exactly.