Neutralising Colour Casts : A Photoshop Tutorial


In a previous post, I’ve described my approach to using filters and how much I love them [Filter Tutorial]. They are however, far from flawless and can introduce many imperfections to an image. One of those imperfections is a colour cast. The most common colour cast I have seen in images are a magenta cast when a graduated neutral density (GND) filter is used. This often results in the sky taking on an unrealistic magenta hue. Sometimes, I like the effect caused by this imperfection and don’t make any attempt to correct it. However, on many occasions, the differential colour between sky and foreground detracts from the image and so, I’ve found a few different ways to neutralise the colour cast. In the example below, I used a 9 stop neutral density (ND) filter while photographing Rannoch Moor (Scotland) to lengthen the exposure enough that the choppy waters appeared smoothened. This particular brand of ND filter (Fader) tends to produce a magenta cast while I have heard that other brands produce a green cast.

This is the reference image exported from lightroom showing the clear magenta cast to the whole image.

Original Image with magenta cast

I’ve found that the following technique most accurately neutralises the unwanted colour cast. The key word is ‘neutralise’ meaning that colour cast is turned into a neutral grey. This technique can be extrapolated to introduce a different cast to the image such as creating warming or cooling filters but that won’t be discussed in this tutorial. The following steps were undertaken in Adobe Photoshop CS5.

Step 1:

Click Layer>New Fill Layer>Solid Colour

Create the filter layer

Step 2:

A new dialogue box emerges. You can select any colour at this stage and rename the layer. I’ve called it ‘Neutralise cast’ for this example. Set the blend mode to soft light.

Creating the filter layer

Step 3:

In the Layers palette, make sure you click off the eye dropper tool so that you can still see your original image with its colour cast.

Click 'off' the layer

Step 4:

Still in the layers palette, double click on the coloured rectangle to bring up an adjustment dialogue box. (yellow in this example)

Step 5:

Hover the mouse over the section of the image that has the relevant colour cast. In this case, the waters of the lake have the most pronounced purple cast. Click on the section of the image and look at the ‘a’ and ‘b’ values as dotted on the screen capture below.

Selecting the colour to neutralise

Step 6:

Invert the ‘a’ and ‘b’ values. In this case ‘a’ changed from 11 to -11 and ‘b’ changed from -9 to 9. You might be wondering what ‘a’ and ‘b’ are all about?

‘a’ represents the colour component of the selection which ranges from magenta to green. Zero is neutral while negative values represent the greener end of the spectrum and positive values represent the magenta end of the spectrum

‘b’ represents the colour component of the selection which ranges from yellow to blue. Zero is neutral while negative values represent the yellower end of the spectrum and positive values represent the bluer end of the spectrum.

With that in mind, it’s no surprise then, that the colour for this layer changed from magenta to green!

Neutralise! Using the 'opposite' colour of the cast

Step 7:

Finally, you may not wish to neutralise the colour cast in the whole image. This is particularly relevant when correcting casts caused by the use of GND filters where only part of the image is affected. For this reason, use the layer mask as seen on the right of the ’Neutralise Cast’ layer. Select a brush [shortcut B]  at a gentle opacity and fill. Set it to paint black and gradually fill in areas of the image you don’t wish to have the colour correction applied.

Selective application of the filter

Final Touches:

As I alluded to in the introduction, this is only one of many ways to correct a colour cast but I have found it to be very simple and accurate. I put the image through the rest of my usual post processing to end up with the following result. Thanks for reading and I hope that you have found some part of this tutorial useful!

Magenta Neutralised

The End Result

-D

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Posted on July 24, 2011, in How we..., Photography, Scotland and tagged , , , , , , , , , , . Bookmark the permalink. 6 Comments.

  1. Thank you for the quick Tut! Keep the good work comming!
    Really good blog!

  2. These are very useful if you use PS, I use a Mac and Aperture but everyone else seems to use PS.
    All the glossy magazines and on line tutorials also seem to use PS and I guess it’s due to the number of people who use PC’s rather than a Mac. Thanks for posting regards
    Ian

  3. Really good tutorial and has proved quite useful, thanks very much!

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