Coloring gray hair requires special coloring consideration since it is often thinner and more delicate. Semi-permanent hair color is less damaging than permanent and lasts about 20 washes. Space your washings apart and co-washing can help to make the color last longer. Use protein treatments and deep conditioning treatments, but not within a week of coloring as the scalp will already be sensitive.
Remember that gray hair loses color more quickly than pigmented hair and is more susceptible to all kinds of damage (sunlight, water, combing). Be sure to use products such as Curl Junkie Curl Assurance Gentle Cleansing Shampoo and beauty routines that will help to maintain color.