There is a very simple and nice idea in UDK documentation about adding color grading to engine in easy way, without writing any special tools. Color grading in UDK uses LUT table (3d texture, with mapping from RGB to RGB). Pretty standard stuff. The interesting part is that this texture is authored in photoshop. LUT slices are added to photoshop layers and game screenshot is set as main layer. When You tweak the screenshot, changes are automatically propagated to layers with LUT table slices. After tweaking screenshot You need just to import authored LUT slices and use them in game. So there is no need to duplicate photoshop functionality and force game artists to work with unfamiliar custom tools.
Moreover we could go further and use HDR source screenshot and HDR LUT table. This way we could add tonemapping with color grading, contrast, saturation… in 30 minutes, without writing any tool code.