Colour Adjustment.

One of the most liberating effects of digital photography is its total control over colour: you can control effects from the most subtle tints to the most outrageous colour combinations.

Curves

Manipulating curves, either all at once or as separate channels, is a potent way to make radical changes on and tone. For best results, work in 16-bit colour, as steep curve shapes demand the highest quality and quantity of image data.

Hue/Saturation

This control globally adjusts hues in the image, as well as the colour saturation and, less usefully, the brightness. You may also select narrow ranges of colours to change, thus altering the overall colour balance. Beware of making colours over saturated, as they may not print out.

Replace Colour

This replaces Hue, within a "fuzziness" setting or wave-band, with another.