Acid staining is the process of using a reactive acid stain to add color to new or old concrete. The acid reacts with the concrete, penetrating the surface and producing a chemical reaction that permanently changes the color. There are many different colors available, ranging from yellows to dark brown and black. Acid staining a smooth trowel finished concrete surface will produce a mottled appearance. Each floor will be unique as the color and appearance can vary widely depending on the finish of the floor and the density of the concrete.