"We highly value our relationship with Ms. Knowles. It is categorically untrue that L'Oreal Paris altered Ms. Knowles' features or skin tone in the campaign for Feria hair color," the Paris-based company said in a statement sent to The Associated Press.
The ad is in the September issues of Elle, Allure and Essence on US magazine stands.
L'Oreal, the maker of Garnier hair care and Lancome cosmetics, is the world's largest cosmetics maker.
A representative for Beyonce said the singer would have no comment beyond L'Oreal's statement.
In the two-page L'Oreal ad, Beyonce's wind-swept hair is a reddish blond shade with highlights. A box of Feria in the ad features a white woman with a similar hair color. Beyonce has been a spokeswoman for L'Oreal since 2001.
The ad created a stir after it was pointed out on Wednesday by celebrity gossip Web site TMZ, which is owned by Time Warner Inc.'s AOL unit.
TMZ's post showed side-by-side photos comparing the ad with a photo of Beyonce with noticeably darker skin. It was the site's most commented on post Thursday afternoon.
A representative for Elle said magazine ads are reviewed before they are printed, but wasn't sure of the exact procedure for checking content.
"We don't put things into our magazine without looking into them first," said Anne Janas, a spokeswoman for Elle.
