Use B&W when the color detracts from the image, or when another aspect -- such as texture -- is interesting.

Tree bark is a good example. It's all brown. But n B&W, you can pump up the contrast and get these amazing textures.

Use color when, well, the natural color is interesting.

Always try out an image as a B&W in photoshop, if you can. By playing with different images in both, you'll get an idea of what you like in B&W vs color.