From my perspective there are a couple of ways to think about color casts. First, many times your overall image may have a specific tint to it. This may be a light blue or green hue. When I see this my first thought goes to the white balance. You have probably either used Auto White balance or forgot to change to the proper preset, or forgot to shoot a custom white balance. On a previous post ("What's All This About White Balance", July 31, 2014) I discussed the specifics about how white balance works and how to correct it so I won't go into it here. Just think about it this way. If there is an overall tint, especially green or blue, then it's probably due to an incorrect white balance.
The second way to think about color casts are times when the light picks up a hue before hitting the subject. This can occur if the light bounces off of a strong color. For example, if the subject is next to a bright red wall and the light bounces off of that wall it will have a red hue. Therefore, the skin color for the subject may have a red tint to it, depending on where the light falls.
So what do you do when faced with this type of color casts? The best way to handle this is to recognize it before taking the photo. Once you recognize it you can either change the lighting setup or move the subject so the color of light being bounced is either much diminished or removed. The problem is that many times we are caught up in the moment that we don't recognize this. So how is the best way to approach this once you go into Lightroom or Photoshop?
There are so many ways to approach this that you would probably get a different answer from every photographer you asked the question. In Photoshop, many photographers use adjustment layers and that's where I am going to focus. My suggestion is fairly simple but takes a little practice. In Lightroom there aren't any layers so I only know of one approach which I will describe later.
First, let's look at the approach in Photoshop. Go to the layer containing the image, then add a Levels Adjustment layer (either within the Layer Menu, or within the Adjustment Layer flyout menu). In the Levels Adjustment select the color channel you want to make the adjustment to. The default channel is RGB which will adjust the level of all colors. So if you have a red cast on the subject you would change the RGB channel to the Red channel.
Change the RGB channel to Red so you are only adjusting that color
Once the Red channel is selected you can adjust the Red by using the three sliders on the histogram. The left slider represents the dark reds, the middle slider represents the mid tone reds, and the right slider represents the light reds. Which one to use depends on the image. The way it works it that it actually adds the complementary color to offset the channel color selected. Therefore, if you have the red channel selected and move the left slider to the right the adjustment will be to add Cyan (the complement to Red) to the dark red tones of the image. You should be careful not to over compensate when you make the adjustments.
Here's a before and after example
Notice the spill of the red color onto the hands and how it is removed in the after example on the right
Making the adjustment will affect the entire image so how do you target the adjustment? Well one of the easiest ways is to use the layer mask. Adjustment layers are automatically layer masks with the entire adjustment revealed. So invert the mask by clicking on the mask and selecting Cmd-I (or Ctrl-I in Windows). Now the adjustment is concealed (filled with Black) and you can just paint in the adjustment as necessary. My recommendation is to lower the opacity of the brush to about 20% and paint the adjustment in slowly on the desired area with a soft brush. This way you have complete control on where and how much of the adjustment is applied.
With a little practice this method becomes second nature. Just be careful not to over do it where the effect becomes noticeable and unnatural looking. You can also do similar approaches with Curves and Hue and Saturation adjustments if you're more comfortable with those.
Now let's look at Lightroom. Lightroom doesn't have layers but it does have the Adjustment Brush. With the Adjustment Brush you can paint in and paint out, much like a layer mask in Photoshop, until you are comfortable with the effect. The primary difference between using the Adjustment Brush in Lightroom and using Levels in Photoshop is that you don't have the RGB channels and the histogram sliders with the Adjustment Brush like you do with Levels in Photoshop. Instead you have to use the White Balance sliders where you can adjust the Temperature (warmth and coolness) of the image, along with the Tint (reds and greens). As you adjust for these colors you should see a change in the histogram to reflect how those colors have been affected in the overall image. Also at the bottom of the Adjustment Brush panel you have sliders that control the Size, Feather, Flow, and Density of the brush. So you are able to paint in or out the adjustment with some control. Getting the effect you're looking for will take a little practice, but everything is non destructive so you can always change anything you have done.
Use the Adjustment Brush to paint in the removal of the color cast
Here's the before and after when using the Adjustment Brush in Lightroom
It's subtle but you can see it in the fingers
Whether you use Lightroom or Photoshop for color cast correction there is a solution for each. Choose the one that works with your workflow and you are most comfortable with.
Next time I'll discuss using the Hue and Saturation Adjustment in Photoshop to accomplish this.
Hope this was helpful. Happy shooting.