Use of heat is necessary for results
As this product does not contain chemicals to open the cuticles. After the colour mix is on your hair. Use medium heat from a hair dryer or heat cap. This opens the cuticles so the colour can go into the hair. Only use heat on the roots or regrowth hair. A heat cap is healthiest. As it opens the cuticles using trapped heat from your head. It also prevents overheating, which dries hair out. Don't forget - USE HEAT!
Men do no need to use heat. Apply. Wait 30 minutes for full grey coverage. Or 15-20 minutes for a 'salt and pepper' look. Rinse.
The heat cap holds body heat and helps to open the hair cuticles to let colour in. Its use does not ensure colour results.