"Does she . . . or doesn't she?" mused the iconic hair-color ad of the late 1950s. These days the odds are she does. About 55 percent of American women 18 and older dye their hair, according to a survey done by Behindthechair .com, an online community for salon professionals.

But while shade ranges and product technologies have improved since the Eisenhower years, the issues that come with coloring -- the fading, the brassiness and the general confusion about which formula to choose -- remain the same.

If you're among the millions who tweak (or transform) their natural shade with chemical help, you've probably pondered at least one of these questions. Here are the answers you've been looking for, from some experts in the field.

Question: Should I wash my hair before I color?

Answer: 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, Mo.

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. "You're looking at a touch-up every four to six weeks," says Amanda Paul, lead color specialist at Salon 124, in Grayson, Ga.

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.

Q: Are hair-coloring chemicals harmful?

A: Dyeing your hair can weaken it, which is why colorists stress the importance of using conditioning treatments and color-specific products. Coloring formulas can also irritate your scalp if it's extremely sensitive, says Nicolay Loor, a colorist at the Stephen Knoll salon, in New York City.

Q: How do I keep my hair color from fading?

A: You can't stop fading completely, but you can slow it down. Don't shampoo for at least a day after you color. From then on, wash only every two to three days, says Jasen James, a color specialist at Chicago's Maxine Salon. Stick to shampoos and conditioners made for color-treated hair, since "these products are designed to keep the hair shafts closed, and that keeps color in," says Crawford. Shield hair from sun with a hat or a color-protecting styling product. Salt water and chlorine can fade your shade, so create a barrier by wetting hair with fresh water, then applying conditioner.

Real Simple magazine

Time Inc.