Chemical stains can be applied to new or old, plain or colored concrete surfaces. Although they are often called acid stains, acid isn’t the ingredient that colors the concrete. Metallic salts in an acidic, water-based solution react with hydrated lime (calcium hydroxide) in hardened concrete to yield insoluble, colored compounds that become a permanent part of the concrete. Several companies manufacture chemical stains that are variations of three basic color groups: black, brown, and blue-green.