I found a nice and not too hard method to correct a color cast on just a part of a photo. In the examples I'm using Adobe Photoshop CS4, but it also applies for CS3, CS2 and Photoshop CS.
Let's take a look at the start image and a close zoom of the head:

The white pole and the shadow around her head have a distinct green color cast. Here's how we can correct this part of the image:
- Create a new adjustment layer Photo Filter and pick a color with the colorpicker which has to be corrected. In this case I took a sample of the white pole.

- Select Hue and add 180 to the current sampled value. In this case the value is 59. So 59 + 180 = 239. Enter this value as Hue.

- Change the density of the filter to 50% (or what works the best). Now we corrected the color cast of the pole, but also for the complete image. It's a bit too cold.
- Add a layer mask and paint everything black where you don't want to correct the color cast and paint everything white where you do want the correction.
- We can repeat these steps on different parts of the photo. I corrected the shadows a bit more and here's the final result and a close up of the part we corrected:

Before & After:

Hope you like the tutorial.