First, open up the image you want to use in PhotoShop. We will be using the following image:
If you have a colored image like this, and you want to re-color it, we need to go to Image > Adjustments > Desaturate, to make it black and white.
The next thing we will do is choose the brush tool, and set its Mode to Color:
Choose a brush size that's adequate for your image size, and then choose somewhere to start and what color you want. You may also want to reduce the opacity of the brush to somwhere around 85%, but again, this would depend on your image, so try different options. I will be starting with the hair, and will paint it brown. If you go over the line and color something you didn't mean to, you can either undo it or use the Sponge tool to desaturate certain areas.
When we're done with the hair, we can move onto other areas, like the skintone. Using color #e9b6a3 for the skin, it's definitely coming along:
Even though the jacket in the picture looks fine with no color, I added a navy blue color over the top of it, and as you are coloring, you will see how much better it looks with color - especially in certain areas.
Now we have a perfectly colored foreground and a grayscale background, which can be left like this for the stand-out effect:
Or we can continue, and color the image entirely:
Using this as a guide, you can tweak the settings to provide different results. Experimenting with PhotoShop is very much advised, as many many results are possible.
More Tutorials: 
