In this Photoshop tutorial we're going to remove most of the blue sky color cast that is visible on a man's white shirt, but what also could have been visible on something like a white wedding dress. I'll show you a Photoshop technique that is flexible and easy to adjust using a Mask and a Hue & Saturation adjustment layer.

Start by making a copy of the background layer in Photoshop, by left clicking on it with the mouse button and dragging it to the New Layer icon ; Photoshop creates a copy of the background layer and makes this new layer active.

Select the Lasso Tool in the tool bar and make a global selection of the man.

With the layer still active, add a mask. Click on the Add Layer Mask icon (1) and a new mask (2) will appear.
Also notice that a new icon () appears in front of the layer (3) that reminds us that we have a layer mask and that the layer mask is active. A layer mask also has a double border if it's active and single one if it's not active.

Make sure that the mask is active. Now go to Filter/Blur/Gaussian Blur... and slightly blur our mask with a small Radius like 4.

Now hold down the Alt key (option key on the Mac) and click on the New Adjustment Layer icon (4). Don't release the Alt key yet! Select Hue/Saturation... in the little window that pops up.

The New Layer window pops up. Release the Alt key.

Select the option 'Use previous layer to create clipping mask' and click Ok. The Hue/Saturation window pops up.

We're now going to change the saturation of the blue cast, by selecting Cyans in the Edit box (5). We now have to select the color cast in the white shirt. Use the Eye Dropper + tool (8) to select as many variations of the blue cast on the white shirt.
Now we slide the saturation slider until we have removed most of the cast. In my example I've used a value of -55 (7). You can remove all blue, but to me the result looks unnatural, so I always leave some blue behind.

Click Ok and we're done.

And here is the final result:

This technique has several advantages, since the adjustments (masks, hue & saturation layer) are non-destructive, the original stays intact. It's easy to fine tune the mask or hue & saturation, making it a perfect technique for several other corrections.
Always make sure that you use selections when you make corrections like these.
There is still some blue cast on the concrete on the right side of the image and on the shirt of the man in the background. These are areas that you might want to include in your mask.

For more help with Photoshop check out oursection.