A concrete dye is a coloring agent offered in a concentrated form. To obtain the desired shade, mix the concentrated dye with either a water or solvent like acetone or alcohol. The water or solvent acts as a carrier penetrating the concrete to change its color. Concrete dye, unlike stain, requires minimal cleanup because it doesn't leave a residue once it's added to the stairs or pathway.
One difference between a concrete dye and concrete stain is the chemicals used. Chemical or acid stains consist of hydrochloric acid, acid-soluble metallic salts and water. The chemicals in a stain changes the concrete's color differently than dye does. Instead of the color penetrating concrete---like dyeing does---staining relies on pigments to change concrete's color. The pigments, or metallic salts, are what prevent the stain from chipping, fading or peeling away from the concrete.
The main difference in using a concrete dye over a concrete stain is the look of concrete. Dyeing concretes creates a monotone to translucent effect. A dye's particles seep deeper into the concrete filling pores, making the color look as if was it is always a part of the walkway or patio. Stain, once it reacts with concrete's hydrated lime, lightly etches the surface. The etching allows the metallic salts in the stain to mark the concrete easier than using a dye.
Time becomes an enemy when dyeing concrete. The dye penetrates the concrete quickly. This leaves little to no time for wiping off excess if too much is applied. Another consideration involves ultra violet, or UV, stability. Most manufacturers recommend dyeing only indoor concrete because the sun may damage the concrete application. If you're considering using a stain to color concrete, know that you have a limited amount of options. For instance, concrete stain options include only earthy tones like terra cottas, tans and blue-greens.