The trickiest part is being able to tell which color goes where.
However, you can use this to your advantage by colourizing pictures with bold colors, creating a striking effect.

Ok, so start Photoshop and open your b/w picture. For this example I'll just use the picture that comes with Windows XP. It can be found in My Documents in Sample Pictures folder. I desaturated it, turning it grey, so I can repaint it.



Create a new layer, and make sure the photo layer is always at the bottom.
Note: When you're repainting pictures of people, it's always good to get some random photo to use as a skin reference. By holding ALT and clicking on the colored photo you can pick the exact color under the cursor. This helps greatly in the repainting process.

So here we go, take the paintbrush tool, by selecting it from toolbar or by pressing B on your keyboard.



Now, choose a brush, I usually use the one you can see below.



But it doesn't really matter that much, as long as it's not some abstract or custom brush.
By now you should have a reference picture open on another layer (or not).
Now take the brush tool, make a new layer, and set the mode to Multiply.



Now using the brush, pick an appropriate color and start brushing areas of your picture.



There we go, by brushing in Multiply mode on a new layer, the underlying picture gets colored. If your painted color is too dark, just use a lighter tone of the same color.

Make sure you maintain this layer layout:
-painted color layer                   Mode: Multiply
-black/white picture layer             Mode: Normal
-(optional) reference picture layer    Mode: Normal

To switch between the reference and the b/w picture, simply press the eye icon on the layer to hide it temporarily.

And, we're done. Get creative!