I'm going to show you a simple method to colorize any picture you want! That is the final image from this tutorial:

Let's go to work. I started with this black and white picture:

Press "Q" on your photoshop to turn on the Quick Mask mode. Or click at it on the toolbox. Select a soft brush on the toolbox.

Make a zoom on your picture (for Zoom Tool, press Z) and start "painting" the arms and body of the girl. You have to paint a part that will have the same color (all her skin). Don't bother if you paint some small wrong part, you don't need much accuracy now.

You should end with something like this:

Now zoom again on the image, and with the eraser tool erase every part that you painted wrong. With more accuracy this time.

After finishing this. Press "Q" again. You'll see a selection of everything you painted! That's what the quick mask do. Go to Select > Inverse. Now Ctrl+C on your Layer(with the selection appearing), create another layer and Ctrl+V.

After the Ctrl+V, the image might be in a wrong position, if so, position it right again. Then, with this new layer selected, go to Layer > New Adjustment Layer > Color Balance.

Mark the "Use Previews Layer to Create Clipping Mask".

Play around with the color balance settings to paint your image. It's a good idea to mark "Preserve Luminosity" on.

I used the following settings:

Shadows: -12 +30 +7
Midtones: +93 +30 -21
Highlights: +27 -7 +20

The skin of your girl is with color now! What about repeating the same procedure for her hair?

Nice hair! I also painted her mouth with this procedure. Also, it's a good idea to delete the part of her "eyes" from the first color layer.

Hover over to see "before" image.

Now you can paint her dress etc. If you want to paint the background of the image (door), don't waste time selecting it. When everything else is already painted the way you want, just do ctrl+click on all layers and then Select > Inverse. You'll have the background selection!

I hope that you enjoyed the tutorial. Thanks