Kim Kardashian, Sugar Ray Leonard, Britney Spears, Ringo Starr. When it comes to choosing spokespeople to endorse its brightly colored footwear, Skechers U.S.A. Inc. has gone for a diverse lineup of celebrities.

But when it comes to picking who sits on the board of the Manhattan Beach, Calif.-based company, things look decidedly more monochrome: nine white men, the youngest of whom is 48 years old.

That homogeneity rankles an increasing number of investors, who are demanding that boards become more diverse. In fact, shareholders have been so successful in pressing the largest U.S. companies to add women to their boards, they're now drilling down into the next tier of businesses and shining a spotlight on diversity—or rather the lack of it—at midcap and smaller members of the Russell 3000 Index such as Skechers.