An image that is not color balanced is said to have a color cast, as everything in the image appears to have shifted towards one color or another. Photos shot under incandescent lighting, as well as older photos, often have a yellow color cast. Outdoor photos tend to have a blue tint. Color balancing, also known as white balance (or grey or neutral balance) is the removal of this color cast.
As with many techniques in Photoshop, there are many was to fix a color cast problem. Here I will discuss several, starting with the easiest and finishing with the most precise white balance correction techniques.
Each method make use of an "adjustment layer" in the layers pallet, which is non-destructive, so there's no need to make a copy of the original image first. You can always come back to an adjustment layer and make changes, since the underlying original image remains intact.
First, click on the "Create new fill or adjustment layer" icon in the layers pallet, or from the menu, select Layer > New Adjustment Layer > Levels
You will see what is known as the photos histogram in the levels window, which represents the exposure of the image when the shot was taken. The first method is simply clicking the "Auto" button in Levels dialogue box. Sometimes this works incredibly well to improve the color balance. At other times it may appear to do nothing at all, but its always worth a try.
Before editing any image, looking at the histogram is a good place to start. Note if there are any "gaps" before or after the shape, as there is on the left side of the above image. Pull in any gaps just into the beginning of the histograms curve. This helps to correct a poorly exposed image.
To properly removing a color cast in a photo we must correct the "white balance" of the image.
Notice three "eye droppers" in the levels (or curves) dialogue box.They are used to select white, grey and black points within in your image.
When there is a color cast in an image, it is as if the image is "confused" about what is supposed to be white, black and neutral grey. We "fix" the problem by showing Photoshop where these points of reference can be found in the photograph with these eye dropper tools.
First, check your settings for the eye dropper tool in Photoshop.
Make sure it is not set on "Point Sample," which only evaluates a 1 pixel square. Here, the settings are changed to a 5 X 5 average pixel sample.
After opening an image in Photoshop, make an adjustment levels layer and select the white eye dropper tool in the dialogue box. Make sure that the "preview" box is checked.
This old Polaroid photo has developed a yellow color cast from age. Select the white eyedropper tool and look for what should be nearly pure white in the image. Then click on that area with the eyedropper tool.
Select the black eyedropper and click on what should be nearly pure black. Like magic, the color cast has been removed!
If we want to improve our accuracy in selecting the whitest and blackest areas in an image, we need a non-subjective method of making the choice. By choosing Image > Adjustments > Threshold, Photoshop can do just that for us. The dialogue box has an image that looks similar to a histogram, and the image turns into a black-and-white preview.
Our first objective is to find the darkest black portion of the image. Begin by sliding the middle slider toward the left.
The image will begin shifting toward all white. Stop sliding just before the entire image turns white (you may want to zoom in on whatever remains black).
As you hover over the remaining black area, the pointer will turn into an eyedropper. Hold down the SHIFT key, and click in the black area. A cross-hair mark will be made in that spot.
This marker becomes the black reference point we will be using later on to correct the white balance.
Now slide the threshold toward the right to find the lightest white within the image. Stop sliding just before the entire image turns black.
As you hover over the remaining white area, the pointer will turn into an eyedropper.
Hold down the SHIFT key, and click in the white area. A cross-hair mark will be made in that spot. This is the white reference point we will be using to correct the white balance.
Important: Click "cancel" (not OK) in the Threshold dialogue box, as we are done with it now.
Now that we have plotted the lightest white and the darkest black areas in our image and make a new adjustment layer (Layer > New Adjustment Layer > Levels), when we click on the white and black eyedropper tools in the levels dialogue box the points we made in the threshold dialogue box are there waiting for us. We can now make much more accurate selections to correct the white balance!
There are times when we need absolute perfection in color balance, and Photoshop has a way. The tool is the grey eyedropper in the middle of the layers adjustment pallet. The only problem is this: we must click on an area in the image that is EXACTLY neutral grey (50% black) to get perfect results.
Start by making a new layer and filling it with 50% grey: on the bottom of the layer pallet click the ‘new layer’ icon. Next, in the top menu go to Edit > Fill > 50% Gray.
In the layer pallet, click the drop down menu containing the layer "blending modes" found on the upper left corner. Change the blending mode, which is set to "normal" by default, to "difference." Your image should now look like a color negative.
The Difference Layer mode inverts pixels containing 50% gray, turning them 100% black.
Select the eyedropper tool and begin hovering over the darkest pixels. Use the INFO pallet and look at the R-G-B values as you hover.
If we can find a pixel that is absolutely pure black (all three RGB channels have a value of 0) we know that is a 50% gray pixel, which is perfectly neutral grey. Finding one will enable use to make an absolutely precise color cast correction to our image. This layer will be discarded when we are finished.
Once you’ve located a pure black pixel, hold the shift key and click the eyedropper tool on the pixel. This adds a target marker, identifying it as a 50% gray pixel. We now discard the 50% grey layer with the "difference" blending mode – we don't need it anymore.
Now we make the correction:
add a "Levels" adjustment layer by clicking the adjustment layer button at the bottom of the layers pallet. When a selection of adjustment layer options appears, select "Levels."
In the levels dialogue box, click the middle eyedropper icon (the "set grey point" dopper) and move the cursor over to your image.
The 50% grey marker we made earlier is there waiting for us. Zoom in very close to be sure the correct pixel is selected. and click on the on the marker we made earlier with the grey eyedropper. The color cast will be removed with absolute precision.