The Asian Development Bank (ADB) is a regional development bank established on 22 August 1966 to facilitate economic development of countries in Asia. ADB is an international development finance institution whose mission is to help its developing member countries reduce poverty and improve the quality of life of their people. ADB's main partners are governments, the private sector, nongovernment organizations, development agencies, community-based organizations, and foundations.
From 31 members at its establishment, ADB now has 67 members - of which 48 are from within Asia and the Pacific and 19 outside. ADB was modeled closely on the World Bank, and has a similar weighted voting system where votes are distributed in proportion with member's capital subscriptions. The headquarters of the bank is in Mandaluyong City, Metro Manila, Philippines, and it has representative offices around the world.
It seems that ADB has a dark blue flag instead of green. Its logo is yellow placed in the center of the flag. Ratio is 2:3.
I found two more photos showing the flag version you reported (see below). Is it possible that the GREEN flag was an older version and the BLUE version is the current ADB flag? I could not find anything on their website about either version.