The Airforce recently changed their uniform coloring and these are the new acceptable items.

ABU stands for Airman Battle Uniforms. They are a sage green color.

Why the change? They wanted to bring in the new camo patterns, add more pockets, use new fabrics, and make them generally easier to care for.

Why the sage-green boots? They're used by other forces, too, like airmen in the Marines. Sage green matches the rest of the uniform, and they wanted a color that wouldn't look scuffed when it's not polished. The new boots are polish-free, so they're designed to look kind of dusty. Black boots always show wear.