The NUPROL Magazine Grip M4-STANAG 2pcs Green is a magazine pull/grip assist accessory available from several online airsoft retailers in the UK. These grips are made from a tough, durable rubber material designed to improve the speed and ease of magazine changes. 
 
Product Information
Functionality: These grips act as a magazine pull assist, making it easier to remove magazines from tight pouches. They can also serve as an improvised stabilization device (for "mag-podding") when shooting from a prone position.
Material: The grips are constructed from a dense, anti-drop, and wear-resistant rubber that attaches to the magazine via friction from internal grooves.
Compatibility: They are designed to slip over most STANAG-style M4/AR15 airsoft magazines.
Color: The specific item requested is green (also referred to as Olive Drab).