Ok, keeping it short and simple. Open the image you want to work with in Photoshop. Press CTRL+SHIFT+U to make it black and white(or Image->Adjustments->Desaturate).
Now, all that’s left to do is select the History Brush tool(shortcut Y) and be sure to not select the Art History Brush by mistake.
Now just go over the areas you want to get their color back and voila:
If you accidentaly color too much of it, you can use the color replacement tool, and set it to Black, to turn areas back to black and white.
An importatn thing to note is that the Desaturate command should be the first and only modification prior to the History Brush tool. If the image was rotated, then the History Brush won’t work. So, if you come across this problem, then firstly color it, and then rotate it, or rotate it, save it, and open it again.
That’s it. Learn to control the brush size and feathering for the best results, and even opacity finds it’s use here. Have fun and enjoy.