Why is the special forces beret green?

Around WW2, select US Army Rangers and OSS operatives were taken over to the UK to complete the British commando course. Upon completion, they were issued the green beret. After the war, these soldiers then went on to become the first members of the newly formed Army Special Forces. They kept the British green berets as a sign of pride, but they were not sanctioned for wear by the Army. President Kennedy saw an exhibition by them and was very impressed, officially authorizing the beret to wear by US Special Forces only. The Army then took a Canadian beret design and created new US Army green berets for the Special Forces soldiers. There's more information on this on the US Army Special Forces Wikipedia page.