Days before a new federal ban on making tobacco products using the words “light” or other terms that might imply a safer smoke, federal regulators on Thursday sent a stern letter to the Altria Group, maker of the cigarettes known as Marlboro Lights.

The Food and Drug Administration asked Altria about notes that have been placed on the last packs of Marlboro Lights reading, “Your Marlboro Lights package is changing, but your cigarette stays the same.” The notes also say “In the future, ask for Marlboro in the gold pack.”

The F.D.A. said it was opening an investigation into the marketing of the cigarettes, which are the nation’s best-selling brand and are sold by the Philip Morris USA unit of Altria.

Starting next Tuesday, tobacco companies are barred from making any products labeled “light,” “mild,” “low” or with “similar descriptors,” according to the tobacco law, which President Obama signed a year ago on that day.

Anticipating the new rules, Altria and other tobacco companies have been phasing out the banned terms. But they have repackaged the same products using what critics call “color coding” — lighter hues of blue, gold, silver or orange for the former “light” products, or in the case of menthol cigarettes, greens.

Through this makeover, Marlboro Lights became simply Marlboro in a gold box and now is called Marlboro Gold.

Although the cigarette companies have said they are using the colors legally to differentiate brands, the F.D.A. has already opened an investigation into the industry color coding.

Now the agency wants to know more about Altria’s effort to tell consumers that light is now gold, by attaching the notes — called onserts — onto packs of Marlboro Lights still in the distribution pipeline.

The F.D.A. told Altria it had until July 30 to produce all documents related to the company’s market research on consumer perceptions about the gold packages and the health risks of the cigarettes inside.

The letter said the onserts “may perpetuate the mistaken beliefs associated with your ‘light’ cigarettes when marketed as Marlboro in the gold pack.”

An Altria spokesman declined to discuss the specifics of the F.D.A.’s query.

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“We just got this letter,” Brendan J. McCormick, the spokesman, said Thursday afternoon. “This is a request for information, and we certainly plan to respond to the letter and comply with the request.”

Separately, the New York City health department on Monday began an advertising campaign to tell consumers that cigarettes in lighter colored packages are still deadly.

“Don’t be fooled,” one television ad says. “All cigarettes contain the same poisons that make you sick and kill you.”