Hiram C. Berdan of New York state created the Sharpshooters, beginning in 1861. The requirements were stringent. The volunteer recruits had to pass a rifle test in order to qualify to be a member of the Sharpshooters. Each man had to be able to place ten shots in a circle of 10 inches in diameter from 200 yards away. He had to possess a keen eye, steady hands and a great deal of training and skill with a rifle. 

The Sharpshooters, like most of the regiments, chose their own uniform. It was green rather than the standard Union blue. The uniform had no brass on any of their buttons to eliminate the possibility that they would be spotted by sun glinting off it. They were one of only a few regiments that went outside the typical Potomac Army uniform. The green color gave the sharpshooters the clear advantage of camouflage, but also something pf a disadvantage because they were easy to distinguish against the rest of the Union soldiers. Sharpshooters were high-priority kills among the Confederate army because they had such high skills.

from Wikipedia