Color theory encompasses a multitude of definitions, concepts and design applications. All the information would fill several encyclopedias. As an introduction, here are a few basic concepts.

Most digital photographers are using level adjustments and/or curve adjustments to adjust and improve their images, but how are curves different from levels, and when should you use them ? This tutorial will help you understand a bit more about curve adjustments in photoshop.

Changing hair color is often simply a matter of adding a Hue/Saturation adjustment layer. However, when the hair needs substantial lightening, such as when converting from brunette to blond, the Selective Color command may be more effective....

Editor's Pick in Photoshop Tutorials, June 2005
Shazam! This professional level tutorial will guide you through the process of creating your very own comic style image with many layers.

This effect is quite popular on forums, its not just as easy as taking the colour away...
1. Open your eye (click image for higher quality version to start with).
2. Desaturate (take out the colour) , by clicking Image > Adjustments > Desaturate...

Change an item's color in a selected picture.

Photoshop lets you choose a color mode for each document. The color mode determines what color method is used to display and print the image you're working on. By selecting a particular color mode, you are choosing to work with particular color model.

Editor's Pick in Photoshop Tutorials, March 2006
Bring color to the dull black and white world! Learn to colorize pictures with our tutorial.

Editor's Pick in Photoshop Tutorials, June 2006
This is a great tutorial that will show you all you need to know in regards to adding color to black and white images. Take some time to check it out. It is definitely worth it.

1. First of all make a new file in Photoshop, I used a size of 210x360.
2. In the new document draw about 20-40 squares and fill them with random colours. (Try get as many colour variations in as possible.)