I have seen on others recommend using hydrogen peroxide as a way to get old urine smell out of carpeting. I don't recommend it. Simply, most household-use hydrogen peroxide has only a 3% dilution which should be safe for colored carpets. However, higher concentrations will bleach dyed fabrics. If you are mixing this yourself, you could ruin the carpet. For that reason, I just don't recommend it. If you really want to try it, be sure to test in an inconspicuous area first.