Female leaders have little issue defining the rules that lead to success in their various industries. But ask them about the rules dictating how professional women should present themselves to optimize their career goals and there’s a good chance your question will be met with hesitation.

Why? For women in the workplace, the whole appearance topic is nothing short of a contradiction-riddled minefield.

They’ve been told attractive people are better off. Thanks to an innate human bias that’s become known as the “what is beautiful is good” effect, beautiful employees are seen as more intelligent, more competent, more everything-that’s-good. Attractiveness has been found to lead to more job interviews, more job offers, higher income and greater overall success.

But wait! That’s not the whole story.

There’s also something known as the “beauty is beastly effect.” Beauty, apparently, can actually work against you. In fact, it can bring you increased discrimination in hiring and on the job, as well as less trust and loyalty when you’re a leader.

“Women are in an impossible catch-22 when it comes to advice on appearance,” says Selena Rezvani, author of Pushback and a speaker and consultant on women and leadership. “Most modern advice on appearance suggests you find a perfect balance between acknowledging your femininity but not overemphasizing it. This increasingly thin line is hard to locate for the average woman, let alone translate and personalize. The short message to women is: your femininity is an asset and liability.”

From makeup to hair color, metrics measuring attractiveness and success are all over the map. Studies in both the U.S. and the U.K. (both sponsored by cosmetics companies, it must be noted) have concluded that the stuff can help you appear more competent on the job. But again, watch out! Wear too much — or not in quite the right way — or just generally look like you’ve spent a lot of time on your grooming, and you might find that you earn less or appear less trustworthy. Going gray has long been considered more of a challenge for women than for men (does it signify experience and wisdom, or just old age?), but even other colors have unspoken connotations. During tough economic times, blonde women have been reported to dye their hair darker so as to be taken more seriously.

“Whether it’s about our appearance or whether or not to assert ourselves, we’re damned if we do and damned if we don’t,” says Bonnie Marcus, president and CEO of Women’s Success Coaching. “It’s a balancing act that ambitious women need to be aware of.”

There’s not an easy solution. “I believe the only general statement you can make about this is that women need to be savvy about the culture of their organization,” Marcus says. As the old saying goes, “dress for the job you want,” she adds.

Rezvani draws two guiding principles from the most successful women executives she’s worked with. First, “they ‘fit in with flair,’ meaning they generally conform to the look of their work culture or industry and—to a lesser extent—demonstrate their own personal style,” she explains. Second, “they round up, not down, in terms of formality, even as the American workplace gets increasingly casual.”

Yahoo CEO Marissa Mayer is a good example, Rezvani says: “Mayer is known for her feminine, modern-chic suits and dresses, a look that telegraphs that she embraces her femininity and appreciates fashion. Yet her ‘flair’ is quiet enough that her clothes don’t detract from her verbal message. She also looks considerably more formal than most in the legendarily casual industry of IT.”

Neena Newberry, president of Newberry Executive Solutions, says women should think about what they want to be known for. “If someone were to describe you, what are the top three things you’d want them to say?” she explains. “Regardless of the rules, figure out who you want to be and bring it to life in language and appearance.”

But ultimately, the most successful women don’t get too caught up in all the attractiveness hullabaloo. It is your inner qualities that set you apart, says Erica Ariel Fox, president of Mobius Executive Leadership and author of Winning from Within.

“We all know people who stand out because of a sparkle they bring into a room,” Fox says. “That’s what people remember—your spark.”