Ahh okay I think I got you now
Try the following:
Make a new layer on top of your B/W image (Click on "Layer > New Layer" when you opened up your image) and set the layer mode of that layer to "Color" or "Overlay" (try around with those a bit):
(Not my image)
Now you can use the Paintbrush tool (with a fuzzy brush) to draw different colors on that layer.
(Again not my image)
By choosing the right colors you can easily create the desired effect (I assume you know how to choose different colors). If the colors are too vivid, try lowering the opacity-slider of the layer to make them a bit more transparent.
By using this method (I used an "Overlay" layer) I received this effect in around 3 minutes:
(from http://www.kottke.org/plus/misc/images/ ... ysburg.jpg
As I said, this was done in a few minutes, and it gets better the longer you work on it.