You can also use the Hue/Saturation command to tint an image. If you are working with a grayscale image, you need to convert it to an RGB image first.
1. Open an image you want to process.
2. Add a Hue/Saturation adjustment layer.
3. Click the Colorize box to tint the image.
4. Adjust the Hue slider to try out different color combinations. Adjust Saturation and Lightness to refine the tint.
The adjustment layer automatically has a Layer Mask attached, which allows you to mask the effect.
5. Click the Layer Mask icon for the Hue/Saturation adjustment layer.
6. Select your Brush tool and press D to load the default colors of black and white.
7. With a small black brush, paint the areas where you want the original colors to show through. so the original red shows through. If you make a mistake, you can press X to toggle back to white for touch-up.
This tutorial comes from the book, Understanding Adobe Photoshop.