I read about this technique a while back and have put it into practice on a number of occasions. Basically, it starts with a problem: there’s a localised colour caste evident in the image. This could be colour castes that are quite unobtrusive but still niggles at the eye (spot from a light of a different Kelvin temperature in a night exposure) to glare that needs to be subdued somewhat.
The goal is essentially to change the colour caste while maintaining the textural detail in the image. To understand how to do this it is imperative that you understand layers and how they work. See my Quick and Dirty Layers Masks tutorial to get an idea of how this works. Step by step, here we go:

1. Create a new empty layer at the top of the layer stack and switch the blending mode to colour (or color if you are American).
2. Select the brush tool (shortcut ‘B’). Choose a relatively low opacity (I like to use between 6-10%). Hold down the Alt key and take a sample of the colour that you would like to match. Paint over the area that is affected by the colour caste until it has been neutralised.

As simple as that! If you want your painting to be more accurate creating a selection of the area that is affected is the easiest way to ensure that there is minimal or no ‘bleed’ of the colour. I find the colour selection tool (Select>Color Range) is very effective in this regard (you can always modify the selection after the initial colour range selection.
Here are some examples where I have put this into action: In the first image of the star trail there is a strong bluish colour caste to the edges of the sandstone gorge. In the second image I have been able to minimise the effect of flare by painting out the actual flare colour so that it matches the colour of the rock rather. The technique is not perfect for dealing with flare, but it certainly helps if you have texture that is not easily cloned or healed. Hover over the images to see the results of the colour layer.

Note that in the second image there is are a few splotches of colour below the sun on the rocks created by flare. By utilising the colour layer it is possible to neutralise the colour. This doesn't eliminate the spot, but makes it much less apparent, if not virtually invisible.

It is important to remember that the point is not to make huge changes, but rather subtlety is the key. In the first image this is particulalrly apparent. Used cleverly the colour layer can be an versatile tool in the Photoshop workflow.