Green comes in many shades. Some greenery isn't even green. Choose a color of green that goes with the majority color in your arrangement. For example a yellow-green leaf goes with yellow and orange flowers. However a dark green color would work as well. What wouldn't work would be a gray-toned leaf. Dramatic colors such as those found in canna leaves require a depth of color in the bouquet. Either combine it with the same color family, such as combining chocolate brown canna leaves streaked with red with chocolate cosmos, or contrast it.
Consider the shape and size of the greenery. Huge leaves such as magnolias would look out of place with a country garden bouquet of small flowers such as bachelor buttons, daisies and marigolds. A better choice would be asparagus fern or lemon leaves. Magnolias leaves look stunning as a ruffle around a nosegay of peonies. Leaves that are long and strap-like, like bear grass, make good accents. Others, such as tropical banana leaves, require tall arrangements to take advantage of the leaf's size and impact.
Greenery that wilts nearly as soon as it is cut and put in water doesn't usually make it in cut flower arrangements. If you're buying the greenery at the florist or grocery, you don't have to worry about the shelf life. However, if you're using greenery from your own garden, cut it and immediately place in water. If it lasts through 24 hours without wilting, it will last in an arrangement.
Most greenery goes with any season. There are a few exceptions. Fall leaves look out of place anywhere except in an autumn arrangement. Holly and mistletoe don't work well except for the holidays. Pine, cedar, spruce, and fir are expected in Christmas arrangements but look odd at other times of the year.