Navy blue hopefully. I say that with confidence based on this article from Selling Power – Design Your Image of Success. From the article (my editing):
While good posture, eye contact, and direct, well-modulated speaking voice have long been deemed important in sales, only in recent years has dress come into its own as an influential factor. And even more recent, says Ms. York, is the realization that colors can also play a basic role in the business of motivating people to buy.
“Colors have a lot to say, and just as sales people should pay attention to the types of clothes they wear, they should also know what colors can say.”
“Salespeople should never wear black, for example,” she says, “because it’s too powerful and overwhelming. However subconsciously, people associate it with death, or the Mafia.”
On the other hand, navy blue is considered the power color, because it is authoritative, like black, but it also evokes trust. Another trustworthy combination is a gray suit with a blue shirt, “just like bankers wear. It commands respect, but it also evokes trust.”
Ms. York agrees with John Molloy (Dress for Success) that solid suits make a man seem more likable than pinstripe suits, and pinstripe suits make a man seem more authoritative. The following are several suggestions for suit and shirt combinations “for the man who want to be well liked”: gray suits with pale blue shirts and maroon ties, beige suits with blue shirts, and dark blue suits with pale yellow shirts.
I’m certainly no slave to fashion so this is valuable advice to me. As we say, much of closing new business comes down to slight advantage. No suit has ever closed a deal, but I bet some attire has cost salespeople a deal.