Over the years, I have had several older muscle cars where I have used Rit dye. It's always been the loop type carpet commonly found in '60-'70's vintage vehicles. And it's always been either black or other dark colors. I just put it in a spray bottle and mist it on, let it thoroughly dry and mist again until you get the desired results. I don't know how it will do with gray, but you could try it out on the section under one of the front seats. I usually add less water than directed, for a high concentratiuon of dye.