3. Fix the Color Cast with Curves 
By making the a and b coordinates 0, we make the color into a neutral while leaving the lightness intact. This is smarter than making up a neutral value for the color. Click to embiggen image
Select Image > Adjustments > Curves to open the Curves dialog. Check the Preview checkbox, and uncheck the Show Clipping checkbox. If the Curve Display Options dialog is not visible, click the downward-facing triangle to make it appear. Make sure it’s set to show the amount of light, rather than pigment, and that all the checkboxes are checked.
Double-click the rightmost Eyedropper icon in the Curves dialog to reveal the Set Target Highlight Color dialog, which sets what the Eyedropper tool will do to the image the next time you click it. The dialog represents the same color in four ways: Hue, Saturation, Brightness (HSB); Red, Green, Blue (RGB); L, a, b (Lab; see “The Power of Lab Color Correction”); and Cyan, Magenta, Yellow, blacK (CMYK). Don’t type anything yet: Doing so is tantamount to guessing, or eyeballing, color, which we’re trying to avoid.
Click the spot you think is supposed to be neutral, to sample its value into the dialog. Now, don’t move the mouse, because we want to keep the cursor exactly at this spot. Press the Tab key until you get to the a field of Lab color, and change a to 0 (zero). Hit Tab again and change b to 0 (zero). Press Return to accept the new target color.
If you haven’t moved the mouse, click the mouse button to cause the new color to be applied to the spot that is supposed to be neutral and to adjust the rest of the picture. If you like the change, press Return to accept it, then tell Photoshop not to save the new target value as a default in the prompt that follows. If you don’t like it, pick a different neutral spot and try the Set Target Highlight Color dialog again.
Once the color is corrected, you’ve got a good starting point for any further work on the image.
The Power of Lab Color Correction 
The L, a, and b values in the Select Target Highlight Color dialog represent Lab color.
There are many ways of arriving at a neutral value. You could just as easily have copied one of the numbers in R, G, and B into the other two values to get a new neutral, but how do you know if you chose the right number?
Lab color solves this problem by separating the color into three different values: L, or lightness, and a and b, which are their positions between red and green, and yellow and blue, respectively.
Think of the detail in an image as differences in lightness. Whether the photo is black and white or color, it’s hard to tell what the picture is about unless it has varying degrees of lightness. By not changing the Lightness of the neutral sample, the color correction is more faithful than it would be if the sample’s lightness were also changed.
Red, Green, and Blue?
Many people learned the three primary colors, which can be used to make almost all other colors, in elementary school art class: red, yellow, and blue. But there are actually two sets of primary colors—subtractive and additive.
The red, yellow, and blue subtractive primaries that the art teacher had you memorize in kindergarten are for pigments, which absorb (that is, subtract) some colors of the light that hits them and reflect the rest back as the colors we see.
When working with light, as with your digital camera, we use the additive primaries: red, green, and blue. They’re called additive because when we project (that is, add) red, green, and blue lights together, they blend to make white light. The art teacher probably didn’t tell you about additive color because you were playing with finger paints, not flashlights
