Gina Armstrong in O magazine

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A recent family photo convinced 29-year-old Gina Armstrong, a resident in physical medicine and rehabilitation at Mount Sinai Medical Center in New York City, that she needed a foundation tutorial. "I noticed a clear line just below my hairline where my foundation ended," she says. "And my whole complexion looked sort of...yellow." So, after a 24-hour shift at the hospital, she headed to the O offices to meet with makeup artist Cynde Watson. Here's what she was doing wrong.