YES, it is authorized. No PIN ON versions with Body Armor.
Yes
The U.S. Army started issuing the Multicam Army Combat Uniform (ACU) to soldiers deploying to Afghanistan in July 2010. The Multicam pattern was chosen as it provides better camouflage effectiveness in the Afghan terrain compared to the prior Universal Camouflage Pattern (UCP). The use of Multicam ACU aimed to enhance the soldiers' combat effectiveness in the specific environment.
On social occasions when the prescribed uniform is the Army Blue Uniform, personnel who don't have one are authorized to wear the Army Green Uniform with white shirt and bow tie.
It came out in early 2010 and was initially issued to troops first going to Afghanistan. There is still not enough for people outside of the war zone to be issued it.
Unless it's specifically authorized by local command, it's against regulations if it isn't part of the uniform.
Units deploying to Afghanistan began receiving them in April 2010, IIRC. Special Forces and other select units had been acquiring them for some time prior to that.
Five
Only the style issued at basic, or a watch cap (think the fishermen from jaws) depending on the uniform.
Uniform items which may be directed or authorized for wear with the basic uniform. Items may be worn with basic uniform at the individual's discretion unless otherwise directed
They wear the same uniforms as the rest of their unit. This may be the ACU or MultiCam uniform for the Army, the MARPAT uniform for the Marines, the ABU for the Air Force.... special operations units have a bit more leniency in what they wear as far as uniforms go. They might wear any of the above, or even the phased out BDU, DCU, or ERDL uniforms if they saw fit to.
ACU'S and some units are starting to get multicam