Dihydroxyacetone or for short DHA, is the primary ingredient in most sunless tanning products.  It is often derived from a plants source such as sugar beets.  DHA simply reacts with the amino acids in your skin.  DHA does not penetrate your skin and studies show that it does not penetrate deeper than your dead skin surface layer.  

In simple terms, DHA reacts with the amino acids in your skin to create a brown or bronze color.  Not all DHA ingredients in spray tan solutions are the same which is why you will see different colors when using different spray tan solutions.  The higher the quality of ingredients in your spray tan solution the better and longer lasting the results.