Luminosity Coloring
This is a coloring technique I stumbled across a few days ago, and after much experimentation I've decided to write and post a tutorial for it. This is a method I've never encountered in my three years of graphic designing.
01. Open your base image in photoshop. I'm going to use this scan of actress Scarlett Johansson from Scarlett-Fan.com.
02. Create a new layer (Shift+Ctrl+N). Flood fill the layer with #B371E5.
03. Change the Blend Mode to Luminosity and the Opacity to 81%. SEE RESULT
04. Select the entire image (Ctrl+A). Copy the merged layers (Shift+Ctrl+C).
05. Hit Ctrl+Alt+Z four times to undo the previous three steps. Hit Ctrl+V to paste the copied image over the original.
06. Change the Blend Mode to Overlay. Merge your layers (Shift+Ctrl+E). SEE RESULT
Now you have a bright, beautiful picture without having to mess with curves or selective coloring!
Other examples:
EDIT: Download the action here! (right-click, save as)