Colorizing a picture can be pretty difficult, so having time and patience is a plus! Firstly, we have to find a good B/W picture, I suggest one that has got a simple background and is not overly complex. There are a lot of ways to colorize pictures, I will show you one of them.
I've chosen to colorize Neve Campbell, because the photo is just wonderful =)
Neve Campbell in B/W
First, we duplicate the Background Layer (right click Background Layer -> Duplicate Layer...). We should have two layers that look exactly the same. Now, we need the Magnetic Lasso Tool, so we can select the skin parts, as shown in the pic:
Just the Skin part
Be sure you select the Duplicated Layer and press the Add Layer Mask button:
Add Mask
Mask Layers: The white parts in your mask are those which have 100% Opacity, Black 0%. From black to white there are different Opacity levels! (From 0 to 100)
Now we add a Gradient Map (Image -> Adjust -> Gradient Map...) She looks quite terrible right now =) Double click the Gradient and you can now edit the gradient. I used a different Neve Campbell pic (which is of course in color) to get the right skin colors for my Gradient Map. The example used here may not have been the best, as the result looks as if it has a Sepia effect. Perhaps you'll have better luck. =)
