If you shoot in B&W, you are limited to the spectral response of the film, corrected by any filter you might use when you shoot the picture. In Photoshop and similar tools, you have more flexibility - you can adjust the brightness of different colors.
Do a web search for Photoshop tutorials on "Convert to B&W". In Photoshop CS3, the conversion tool is especially good. Since a picture is worth a thousand words, it's best for you to look at some of the examples that are already out there. It's enough to make you seriously consider shooting with color film all the time.
Here is a link to a short video that shows it in action.
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