Permanent hair dye is the most effective for covering gray hair due to its ability to penetrate the hair shaft and alter its structure. This type of dye contains ammonia and hydrogen peroxide, which can damage hair if not used properly. Demi-permanent and semi-permanent dyes are less damaging and provide more temporary results, lasting 12-24 washes and 4-6 weeks, respectively. Temporary dyes, such as foams and sprays, can wash out after one shampoo. For natural-looking coverage, choose a color that matches your natural base and apply it according to the instructions.