Ah … now I can tell them apart!

Such was the sigh of relief heard around the world when "Hilton sister" Nicky Hilton (search) and "Olsen twin" Mary-Kate Olsen (search) dyed their hair brown, apparently to differentiate themselves from their famous blond siblings.

Pop star Jessica Simpson's (search) little sister Ashlee has jumped on the brunette bandwagon, too, seemingly to avoid becoming known as a "Simpson sister." But experts say individuality is only part of the reason why these famous kin are hitting the dye bottle.

"When people go from blond to brown, they want to be taken more seriously — especially in Hollywood, where there are so many blondes with long hair," said Celebrity Hairstyles magazine editor Mary Greenburg. "Everybody looks like Jessica Simpson and Britney, and you get pegged into that pop image."

Us Weekly news director Marc Malkin agreed that these brown-haired girls are doing more than asking the world not to confuse them with their famous sisters — they're also trying to say something about their personalities.

"The ones who do dye their hair are the ones that are not the more ‘blond' of the two, if I may say so," Malkin said. "Jessica Simpson's sister Ashlee is more the rocker chick, more indie. Nicky Hilton is a little more subdued, more the quiet type, more brunette than Paris."

Indeed, "7th Heaven" actress Ashlee Simpson's recent dye job coincided with the release of her folksy-sounding hit song, "Pieces of Me," and the debut of her MTV reality show, "The Ashlee Simpson Show," which is documenting her attempt to become a singing star.

Mary-Kate Olsen, who colored her hair around the time of the famous twins' 18th birthday and the release of their movie "New York Minute," is known as the more bohemian of the two sisters, while Ashley is considered the more glamorous. (Mary-Kate was recently hospitalized for health problems.)

And Nicky Hilton, whose dye job took place around the same time as the public release of her hard-partying sister's sex tape, is also seeking to be known as the more reserved, professional half of her pair.

"Nicky Hilton's agent told me she wants to be taken more seriously — she wants to be a TV host. Brown hair can convey that image more," said TV producer and fashion expert Elycia Rubin, author of the upcoming book "Frumpy to Foxy in 15 Minutes Flat."

Rubin says it's silly, but people really do perceive blondes and brunettes differently.

"Brown is a more sophisticated look — it goes all the way back to 'blondes have more fun.' Some of the most brilliant women are blond, but there's a 'dumb blonde' stigma attached to it," she said.

New Yorker Kristi Nucatola, 27, a brunette who frequently colors her hair blond, red and even fuchsia, thinks dying your hair any color is an attempt to stand out in the crowd.

"I don't have a sister, but even me – when you step outside of your normal skin, you feel like you differentiate yourself from the rest of the world anyway," she said. "It's like an actor stepping into another skin for a part."

Nucatola added that she doesn't think a sister who goes brown is seen as more introspective or serious than her golden-haired sibling. On the contrary, she believes that sister is viewed as the more free-spirited of the two.

"She's just the one who's more willing to make a change," she said.