How to Use Gradient Maps in Photoshop CS6
In Phtoshop CS6, gradient maps convert your image to grayscale and then replace the range of black, gray, and white tones with a gradient of your choice, in effect colorizing your image — often, in startling ways.
Photoshop maps the lightest tones of your image to one color in the gradient and changes the darkest tones to the other color of the gradient (assuming you’re using just two colors for the gradient). Photoshop changes all the formerly gray tones to an intermediate color between the two. When you use multiple colors or fancy gradients, the image gets interesting. Just follow these steps to try out this feature:
Open an image and access the gradient map, by choosing Image→Adjustments→Gradient Map.
Choose the gradient you want from the gradient list.
This list is exactly like the one offered with the Gradient tool. You can edit the gradient used for your map exactly like you do for the Gradient tool.
Choose either or both of these options:
Dither: Adds random noise to smooth out the gradient and reduces banding.
Reverse: Changes the direction of the gradient. Use the Reverse option to create a negative quickly.
Click OK to apply the gradient map.
If the effect is a little too intense for your taste, try fading the gradient map (Edit→Fade Gradient Mask) and then adjusting the opacity percentage and/or applying a different blend mode.