When I was growing up, learning photography was pretty much an exclusively black and white exercise. A color darkroom was prohibitively expensive - and complex - and black and white film was much cheaper than color. Nowadays, for the price of a $100 photo printer, you can have a high-quality color lab on your desk, which outputs at a far lower per-print cost than a color darkroom, and with none of the hazardous waste disposal troubles. The downside to this is that most photographers now start learning in color.
I say “downside” because there are great advantages to being forced to shoot in black and white, from the biological to the aesthetic. Your eyes, with their preponderance of luminance-sensitive rods, are much more responsive to changes in brightness than they are to changes in hue. Your eyes separate the color and luminance information they receive into separate signals and transmit each of these to the brain via the optic nerve. The luminance signal contains all of the fine spatial detail that you perceive, and this signal contains much more information than the lower-resolution color signal. Consequently, black and white prints actually contain most of the information that the brain uses to construct a scene. This is one reason that black and white prints can be so satisfying - they have all of the essential information that the brain depends on, with no distracting color data.
All photographs begin with light, of course, and learning to recognize and use lighting is an essential photographic skill. Because black and white prints are composed entirely of luminance - changes in lightness - shooting in black and white helps develop your sensitivity to lighting and form, light and shadow.
Very often, a scene that is boring or cluttered in color will be compelling in black and white. Because the real world is in color, learning to visualize in black and white can be difficult for the inexperienced black and white shooter. That is, it can be difficult to recognize that a potential image might work in black and white.
For example, in the figure below, the image on the left - which is roughly how it appeared while standing on the street - is not especially interesting. When presented as a black and white image, though, it’s much more evocative and compelling.
Your Aperture library can be a great way to practice black and white visualization. Pick a project of color images and try to visualize those images as grayscale. If you find any that you think are particularly good black and white candidates, create a new version (select the image and press Option-V) and then apply Aperture’s Monochrome Mixer. Don’t worry about adjusting any of the parameters, just take a look at the image in grayscale and how you like it.
As a form of practice, this process can get you “thinking” in black and white, in much the same way that you will when you go out in the field. What’s more, because the images in your library are, presumably, images that you’re pleased with, you’ll know you’re already working with images with good composition. Of course, what you’re not necessarily getting are the images that you didn’t shoot in the first place, because you weren’t thinking in black and white while shooting. Hopefully, this practice will help you better-recognize potential grayscale scenes the next time you’re out shooting.