Fixing Colors in Photoshop


Color casts occur in images for at least three reasons:
  • The lighting did not matching the color of sunlight, as happens with artificial light indoors
  • Sunlight is reflected off a strongly colored surface onto the subject, as when junior is tinted green by light reflected from the grass
  • The image is derived from film or a print that has suffered a color change over time
The colors can be adjusted in Photoshop to improve the colors, and often correct it completely.

Cameras have "white balance" adjustments that compensate for the color of the light source when the picture is taken. If you can operate those controls quickly, it's better to make the adjustment when the picture is taken. We obey the Quick Shot principle that it is better to diddle on your computer than on the picture taking spot.

Here is a scan of a photograph taken on film in 1998 and later digitized.  This was a memorable dinner in Sapporo, Japan with sushi so fresh some was still moving. It was a great meal, well worth a good photo. We have started to clean it up by adjusting the levels and cropping it, but the colors are still not right:

Original Color Tint

In Photoshop, color correction begins by selecting  Enhance > Adjust Color > Remove Color Cast and then clicking on the eyedropper icon that appears in the dialog box. The dialog reminds you to use the eyedropper to select a color in the scene that should be a shade of gray.  Photoshop will then correct the entire photo so that the point you selected appear neutral gray. The question is what point to select. The dishes should be white, but that is not a good choice. The dishes are overexposed and anything overexposed with always be white, no matter if it was purple in real life. The shadows are not really black, they are a dark shade of the color of the object in shadow. A good choice is this spot, bottom center, circled in purple:

Circle spot

We don't know for sure that the paper in the circle should be white or gray, but it is a good guess. When you are taking a picture that may need correction, you may need to put a small piece of paper near the edge of the image to have something to use for correction. Usually there is something in the image. Clicking on the selected spot with the Remove Color Cast eyedropper yields:

Corrected for gray

This helps quite a bit, but the dark areas have a bluish cast.  The bluish cast is probably an artifact of the film exposed in low light. One could try to find other sample spots, but in this case other points make the picture look worse. Instead, the blue can be reduced manually. The most effective way of doing this is to Enhance > Adjust Lighting > Levels.  This brings up the histogram of the gray levels in the image. Near the top of the dialog box there is a drop down menu that comes up as RGB.  Drop down the other choices and select Blue. This changes the graph to the shades of blue, with dark shades on the left. We have too much blue in the image so we want to make the shades darker. Moving the arrow that marks the center to the right dims the blue. A small change removes the bluish cast:

Blue adjusted

This is not perfect, but it is a significant improvement over the original image.
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