Red Cyclamen, photo by Harold Davis. View this image larger.
Where the composition of a photo is about pattern, a single color often plays an important role. Cases in point: the photo of the cyclamen above is almost entirely red, with green accents, while the capture of water drops on a lupine (below) is essentially monochromatically green.
Related links: A Sense of Scale; Patterns category on Photoblog 2.0.
[Both photos: Nikon D300, 200mm f/4 macro lens (300mm in 35mm terms), and 36mm extension tube. Above: 3 seconds at f/36 and ISO 100. Below: 1/3 of a second at f/40 and ISO 100.]
View this image larger.
