In composting, browns are carbon-rich materials and greens are nitrogen-rich materials. Browns are usually dry items such as dry leaves, plant stalks and twigs, shredded paper or cardboard, and untreated wood chips. Greens are typically moist or fresh items such as fruit and vegetable scraps, grass clippings, coffee grounds, and crushed eggshells.
The labels refer to nutrient roles, not just color—for example, coffee grounds are counted as greens even though they look brown. The U.S. EPA recommends adding at least two to three times the volume of browns to greens so microbes get a workable carbon-to-nitrogen balance, plus moisture and oxygen, to break the pile down into finished compost.
Source: U.S. EPA — Composting At Home.