The answer is seemingly simple: if color is important to an image, it should remain in color. If color is not important to an image, it should be converted to black-and-white. But it is actually a bit more complicated than that.
For color images, one must think in "color." One must actively look for color play. One must understand color theory, and if there are contrasting or complimentary colors at work, and then use that to one's advantage. Actually, it doesn't have to be that complicated. I think people can generally find good color situations, if just from a gut instinct. But it takes practice and vision to consistently create great color images.
Monochrome images are much different. One must actively look for light, contrast and design. Black-and-white is a whole different ballgame than color. What makes a monochrome photograph great is different than what makes a color image great.
The point here is that one must consider before even capturing an image whether or not it will be color or black-and-white. After deciding, you can then make a strategy how to best capture it.
I recently captured the photograph below, and made a monochrome version and color version of it. I intended the photograph to be black-and-white because I thought it would best show the design created by the mower tracks. I also liked how the grass contrasted with the background mountains. After completing the monochrome version, I decided to make a color version because the yellow grass contrasts with the blue sky and shadowed mountains. In the case of Mowed Field it works as both a color and black-and-white image.
|Mowed Field (Monochrome Version) - Stallion Springs, California|
|Mowed Field (Color Version) - Stallion Springs, California|
Which version is better? I like the monochrome version myself. I think it is more dramatic, and fulfills the original vision of the photograph. The color version is good, too. I find it interesting how the two images show the same scene in much different ways. The eyes are even drawn to different places in each version.
Even though Mowed Field works well as color and monochrome, the scene has a different feel in each version. I have to decide what I want the viewer to take from my image and use that as a deciding factor. In this case, the color version just doesn't quite convey what I wanted to convey to the viewer, so the black-and-white photograph wins.