Real World Comparisons:
Poplar tree bark
Streaky dye job
Over-clorinated hair (swimming pools)
Cut kiwi fruit
Golf course greens
Tree moss (Old Man's Beard, etc)
Safety stripes/Safety clothing (police/firemen/etc)
John Deer products
The Incredible Hulk
Menthol cigarette packs
Military vehicles (tanks, jeeps, etc)
Shades of Green:
Make every detail count
Colors are powerful descriptors, not fillers. Make sure that if you use a comparison or contrast to highlight a color, you choose the right one. Look at the setting and atmosphere you are working to create, then draw from the viewpoint character or narrator's history, education and past experiences to find the right fit.
A poor example:
When Jasper said his cousin was comin' to visit, I figgered he'd be like the rest of the farm boys: sturdy and dumb as an ox. Boy, was I wrong. His hair was raven black and his skin was fair--you could tell he weren't the type to spend long hours in the sun. And his eyes! Green like emeralds sparkling in the showcase window at Tiffany's.
What's wrong with this example?
The style of writing here suggests the character has minimum education and life experience. It's doubtful he would liken someone's eye color to a high-end, big city jewelry store product, right?
A strong example:
I didn't know what to expect--I mean, I'd never seen anyone poisoned before. They said she had survived, but now after seeing her, I wasn't sure. The hair I'd always envied was straight and dull, like the curl had been sucked out of it. Her lips were as thin and flaky as a waffle cone. And her skin--not only was it pulled tight over her knobby bones, its color was wrong. You know when water sits in a ditch for awhile, it gets that layer of greenish scum floating on top? I covered my nose, in case she smelled like that, too.
Why is this example better?
This example works because the character's thoughts lead to a comparision that creates a vivid and unpleasant image. This detail fits perfectly with the conflict (a poisoning) and supports an atmostphere of erosion and decay.