Checking Sky Blues
If you don't have any obvious Greys to work with then the next best Key colour is Sky Blue. The same basic method will be used here as was done with correcting neutral greys in the previous section.
If you don't know about the 'Key Colours' then read the section on Key Colour Patterns.
Reading the Image
A colour cast should be a little bit more obvious in this example as can be seen in the slightly purple clouds. There are Grey areas that could be checked, but if you look at the blades on the wind turbine you will see that they have a deffinate Yellow colour to them, This indicates that the photograph was taken in the late afternoon, but before sunset, so it is a warning that the Greys in the image may not be neutral as the White light is failing.
There is blue sky though, which will remain blue up until sunset.
Clicking on the blue sky with the Eydropper from the Toolbar shows that the sky Key colour pattern is incorrect. The Red is too high, and the Green is a little low.
Balancing the Blue Tones
Leave the Colour palette open (it will be needed while the colour correction is being done) and open the Curves panel.
REPEAT WARNING: The sliders on the Colour palette need to be "active", meaning that they must move while the colour correction is being done as they will tell us what to do and by how much. After the Curves panel has been opened you MUST click again on the blue area being measured. If you don't do this the Colour sliders will remain static.
From the Channel drop-down list, at the top of the Curves panel, select the Red channel. Click on the middle of the curve and pull it down, watching the sliders in the Colour palette as you do so. Stop when the Red slider has comes to roughly 45° behind the Blue.
Now select the Green channel, and this time raise the curve a little until the Red, Green and Blue sliders are roughly in a straight line.
The finished image now has the purple cast removed.
Finally, looking at the Colour palette again and clicking on the blue sky with the Eyedropper Tool you will see that the colour pattern is now as it should be.