Here's the technique I use: I take ALL my photos in color. Period. Some cameras (mine included) have a B&W mode which removes the color for you, but I have a couple of problems with this in-camera method.
1. The camera is making the decision of what areas should be white, what areas should be black, and what grey levels to display in between. I prefer to make that decision myself.
2. You can always remove color from a color image, but you can never add color to a B&W white image. If the image was captured as B&W using this in-camera method, the color information is gone forever.
I prefer instead to use software to turn the color image into a B&W shot. There are several ways to do this. Probably the most commonly-used and simplest method is to simply adjust the saturation level to zero. This is a fair approach but usually isn't adequate for nice portraiture work because the skin tones are usually a bit dull. Just my personal opinion.
The numerous other techniques available when using software such as Photoshop are far beyond the scope of this discussion thread. But now you have a better idea of what you can do.
After de-saturating, be sure to perform a "save-as" instead of a "save" so you don't lose your color version!