Concrete stain actually reacts with carbon dioxide in the air to become harder. The process is called carbonation - just like soda pop. It fall in to two main categories, which includes solid color stains typically called concrete stains or pigmented sealers and transparent or reactive stains better known as acid stains. Solid color concrete stains impart an even, solid look to the floor and come in over 20 colors. Acid stains provide a transparent marble like look to concrete, the colors are more limited and go from shades of blue-green to dark brown earth tones. The other option includes concrete dyes, which provide transparent colors and accents in bright hues that help achieve results not possible through acid staining.