Turkey's national airline has barred female flight attendants from wearing red lipstick and nail polish.
The move has disturbed secular Turks, who fear the country is becoming more Islamic.
Turkish Airlines said the ban was aimed at keeping crews "artless and well-groomed with makeup in pastel tones", as a natural look improved communication with passengers.
"As a consequence of our current cabin uniforms not including red, dark pink, et cetera, the use of lipstick and nail polish in these colours by our cabin crew impairs visual integrity," a statement said.
Turkish Airlines, Europe's fourth-biggest carrier, declined a request for further comment.
The guideline follows other restrictions on employees' appearance and on serving alcohol.
Critics say they reflect the influence of the government's conservative religious values at the fast-growing state-run airline, one of Turkey's most recognised brands.
"This new guideline is totally down to Turkish Airlines management's desire to shape the company to fit its own political and ideological stance," Atilay Aycin, president of the airline's Hava-Is labour union, said.
"No one can deny that Turkey has become a more conservative, religious country", he said.
Turkey is 99% Muslim, but the NATO state and European Union candidate has a secular constitution.
Prime Minister Tayyip Erdogan's ruling AK Party, which traces its roots to a banned Islamic party, has relaxed the state's control over the expression of religion.
One of the measures he has relaxed was the once-strict limits imposed on wearing the Islamic-style headscarf.
Such restrictions were aimed at reining in Islamism and improving women's rights.
It effectively prevented many devout women from studying at university or taking government jobs.
Turkish Airlines scrapped its own ban on the headscarf more than a year ago.
Covered women now work at check-in counters and at other positions in the company, Mr Aycin said.
Other Turkish carriers also have guidelines on the appearance of cabin personnel.
The flag carrier caused a stir earlier this year when newspapers published mock-ups of a new Ottoman-style uniform for stewardesses with ankle-length dresses.
The airline's management has appeared to have since abandoned this proposal.
That was followed by a ban on alcohol on planes flying to most domestic destinations and some Islamic countries.