Q. Should I wash my hair before I color?

A. No. The natural oils on your scalp act as a buffer between your skin and the chemicals you’re applying, so it’s best not to shampoo for 24 hours before coloring. If you’re going to a salon, make sure your last wash before the appointment isn’t with a color-depositing shampoo or conditioner. These can skew how the colorist perceives your current shade, therefore altering the results.

Q. What’s the difference between semipermanent and permanent color, and how do I choose?

A. Semipermanent color contains pigments that coat the outside of the hair shafts. It washes out in about five to seven weeks, so it’s good for people who are wary of a big commitment to color or who want just a subtle change, says Shayla Crawford, lead colorist at Astrids Day Spa, in Springfield, Missouri. It can darken the tone of your current shade and cover a few gray hairs, but it can’t address a full head of white strands. Permanent hair color, on the other hand, typically contains ammonia to open the hair cuticles, so color gets deposited inside the shafts. It may also contain peroxide, which bleaches out hair’s natural tones. A permanent dye job can lighten hair, completely change its shade, and fully cover gray, but maintenance is high: Your roots will be obvious. “You’re looking at a touch-up every four to six weeks,” says Amanda Paul, lead color specialist at Salon 124, in Grayson, Georgia.

Q. What factors affect how my hair takes color?

A. The two biggies: your hair’s health and its texture. Healthy hair holds on to color best. If yours is dry or damaged, it might fade quickly. Finer hair lightens faster and generally requires more gentle (semipermanent) agents, while coarser hair is more color-resistant but can handle potent chemicals, says Kim Lundin, creative director of the Gene Juarez Salons and Spas, in Seattle. At a salon, a colorist can make adjustments to accommodate your hair type by leaving the color on longer or washing it out sooner, but at home it’s harder to make those judgment calls. (That’s why salon color is often a safer choice.)

In addition, some medications and other drugs may affect hair’s receptivity to dye, says Richard Sollazzo, an internist and oncologist in private practice in New York City. The most common culprits could be thyroid medications, chemotherapy, and some types of hormonal medications. Always alert your colorist about medication changes.