Think in Black and White, Shoot in Color
Black and white photographs can appear dramatic, replacing the distraction of color with an emphasis on contrast, textures, and form. But all too often, all you end up with is a lifeless, flat monochrome image. It's not as easy as it seems.
What we see around us can be characterized in terms of hue and tone. "Hue" refers to the color component, while "tone" describes the luminosity or brightness. When we plot this relationship as a graph with each factor along one axis, it becomes evident why black and white can be challenging. To create a black and white photo, you have only brightness to work with, kind of like having one hand tied behind your back.
It's easy for two colors to appear equally bright. Place a bright red flower in front of some green vegetation with equivalent luminosity, and the two can easily become indistinct and appear as one when converted to black and white. In color, the red will stand out clearly from the green. Remove the hue, and the flower will all but disappear unless you look closely for its outline. To create that dramatic appearance you are looking for, you need some way to separate flower from foliage by more than just hue.
The traditional solution to this problem back in the days of film was to use colored filters in front of the camera lens. The only way to record a black and white image was to use black and white film, so any modification had to be made before the light reached it. Place a filter that only passes red light in front of a red flower, and most of the light will make it through. That same red filter, though, won't be as kind to our green vegetation. Plants aren't entirely green, but if they were, they would appear black. No light reflecting off them would be able to survive the filter. Through the use of a filter, photographers could re-introduce the distinction between flower and background lost by removing color.
Different colored filters produced varying effects, but luckily, you could see it through the viewfinder. Green filters would make foliage lighter. A pale green filter could even out skin tones and hide some blemishes. An orange filter will darken the sky since it would pass so little blue. Yellow filters could improve contrast and were popular when shooting landscapes. You get the idea. All you had to do was pick the right colored filter and shoot as normal.
Many digital cameras feature a black and white mode to make old-time film shooters feel at home. While this will work, I would advise against it. If you've never thought about it before, camera sensors already see only grayscale. To allow them to record colored images, they come equipped with green, red, and blue filters as part of the Bayer array technological marvel that makes all this possible. The Bayer array would further filter any light making it through a red-filtered digital camera lens. Pixels with a green or blue Bayer filter would be rendered useless. The resolving power of your expensive camera sensor would be significantly diminished, with only red photosites able to record much detail. Thankfully, digital provides a better solution.
Most major photo editing programs can convert color images to black and white via software, effectively replacing the glass or acrylic physical filters of earlier generations with digital ones. Program sliders let you selectively filter individual portions of the color spectrum while monitoring the effect on the biggest computer monitor your desk and budget can support. Cool.
Digital gives us more control over the process than film did, and we get to defer the task of conversion so we can focus more on composition and other matters while in the field. I like having the ability to custom control the strength of each filter color to balance multiple needs. And if I need more control still, there's always masking to filter portions of the frame differently.
But all this luxury doesn't mean you should forget about black and white entirely while shooting. As you gain experience with how various colors interact, it helps to try thinking in black and white from the beginning. Just because I can spend hours playing with layer masks and assorted means to get the look I want, I've found it works better if I can compose the shot to avoid such herculean measures.
You can help visualize luminosity variances by looking at a scene while squinting. As you gradually close your eyelids, portions of the frame that fall to black simultaneously must reflect similar amounts of light. But it's as straightforward to see potential opportunities for exploiting color contrast. Practice helps, but your best place to start would be to consider the color wheel. With the colors of the rainbow arranged around a circle, pigments opposite each other will cancel each other out. Red will be opposite green and so on. The closer two colors are to each other around the wheel, the more they will shift together when subjected to a single filter color. The further they are apart, the easier it will be to separate them satisfactorily via filtering.
First, examine the scene by squinting, searching for areas that may cause problems. A foreground and a background with widely varying luminosity will translate easily to black and white. But if too close together in brightness, you should consider how you can filter them to alter how they will render on your computer. Sometimes, all it takes is a slight change of composition to avoid a conversion nightmare entirely.
So while it is possible to shoot digitally with no thought of black and white, you can make your task more enjoyable and productive by not forgetting it altogether. Shoot in color, but think in black and white.