CORRECTING COLOUR CASTS
Most digital images, whether from a camera or from a scanner, have a colour cast.
Photoshop has a colour correction tool called Colour Balance ( Image > Adjustments > Colour Balance) (shortcut CTRL + B).
This is OK to use if you know what the colour cast is, i.e. is it a magenta cast or a red cast or somewhere in between?
To take the
guesswork out of correcting colour casts the mid
grey point eyedropper tool within the curves dialogue box can be used.
Image > Adjustments > Curves (CTRL+ M) To bring up the curves box.
Click on the middle eyedropper and then click on your image at a point where the image looks a medium grey colour. This should correct the colour cast.
If it looks wrong or you are not sure which point is a medium grey then there is a way to pinpoint exactly where in the image the mid grey is.
Go back in History
to the uncorrected image then in the layers
palette click the Create a new layer icon.
Fill this layer with grey, Edit > Fill, and choose 50% grey from the drop down Use box. Click OK.
Back in the Layers palette change the Blend Mode to difference.
Click the
Adjustment
Layer icon at the bottom of the palette
and choose Threshold.
Move the slider all the way to the left then slowly start to move it back to the right until the first area of black appears. This is the mid-tone grey in the image. Click OK.
Select the Colour
Sampler
Tool (under the
eyedropper tool) from the toolbox and change the sample size to 3 by 3
average.
Click on your image
on one of
the black areas (neutral mid
tones) to mark it.
Drag the layer mask and the grey layer into the bin as they are no longer required.
Open the Curves dialogue box and using the midpoint eyedropper click on the target area (the area you have just marked) to correct the colour cast.
The target can be removed by clicking Clear.
If this does not quite do the trick then open up the curves dialogue box again and select the left hand (black point or shadows) eyedropper in the curves palette and click on the darkest area of your image. Next, select the right hand (white point or highlights) eyedropper and click on the lightest part of the image. Using these droppers should also correct any exposure problems in the image as well as correcting the colour cast.