1. Take a picture of your choice, open it in Photoshop and make it any size you want. Now create a new adjustment layer of Hue/Saturation. Now press D then X and fill the Hue/Saturation Mask changing it from completely white to completely black, however this will not affect your image yet.
2. Now with a Hue/Saturation Adjustment layer mask, when you color on it, lighter areas are the most effected and conversely darker areas are the least affected. Now double click on (shown in red):
a new window should popup. Check the box that says colorize, and drag the Hue and Saturation bars till you get your desired color.
3. This is the fun part. Select your paintbrush, non faded edges is what I used. Make sure white if your foreground color, if not, then press D then X. Now with the Hue.Saturation 1 layer selected, start painting on the parts if the image you wish to change the color of. What you are doing is painting white on the mask, letting some of that color through. You can slowly make the brush sizes smaller for the more fine detail. Here is what my layers looked like at then end.
There you have a nice easy way to add color to your image. Now if you don't want the color to be as strong, use different shades of gray instead of white.