Especial attention to color is paid by abstract photographers. Many seek color harmonies, focusing on the symphony of similar colors in the highlights and lowlights of hair, or rust stains on old cold iron.
Others still seek to point out the exact same hue of entirely unlike objects. A bright green grasshopper may be juxtaposed next to someone's eyes, or a wedding ring next to a doorknob. Still others try to show complete contrast.
The use of color filters is a very common technique. Rainbows may be splayed across otherwise color-drained photographs of cities, or the undercanopy of a forest turned to a distinctly underwater shade of aquamarine.
Similarly, color isolation is another technique often utilized. A girl in a bright red dress in a blurry black & white city, a single leaf on a tree remaining green.
Of course, there are countless other color techniques and effects pioneered by abstract photographers, many of which have since moved into other fields of photography.