Question: how much 20 volume developer to mix with hair color?
Answer: If you want to lighten your hair you can use 20 volumes and can be used for gray hair coverage. However, using 20 vol slightly damage the hair strands. Mix 50/50 between dye with developer 20vol to get 15vol suitable for fine gray hair coverage, 20 volumes 6% H2O2 with standard gray loose on long-lasting gray hair.