What Photoshop's Colorize option in the Hue/Saturation adjustment window does

Photoshop Colorize

The Colorize option changes the nature of the Hue/Saturation control.
When checked, it removes the color from an image and overlays the image with a tint of a single hue and saturation.
Each pixel's luminosity remains unchanged (actually, it is changed, but very little).
With the Colorize option, the Hue and Saturation values are no longer relative numbers based on an offset from a starting point.
Instead, they are absolute numbers.
The Hue value ranges from 0° to 360° and represents an absolute position on the color wheel (0° and 360° are the same color; red)
The Saturation value ranges from 0% to 100%.
If Saturation is changed to 0%, color will be negated and the pixels will go gray.

Using Hue/Saturation to colorize an image is not the same as making a duo tone.
A duo tone is a grayscale image where two different colors have been applied to different parts of the image's tonal range.
The colorize option of Hue/Saturation applies only one color.
However, similar to a duotone, it does it in a non-linear manner.
When using Hue/Saturation to colorize an image, whites and blacks will not be colorized, mid tones will have the most color applied and the shadow and highlight areas will show the least.
Think of using the Colorize option as the traditional black and white darkroom technique of toning a print.

Credit for the above goes to: http://www.zuberphotographics.com/content/photoshop/hue-saturation.htm