Colleges and universities do seek to build diverse populations. They'll look to your admissions essay to discover just what makes you different from all those other hopefuls in the pile of applications. Race, creed, and color are characteristics that you share with other peoples. The goal of your essay is to distinguish yourself from everyone else, making the unique you a memorable candidate.
Your ethnic origin, religious beliefs, or cultural background are certainly not off limits — if you're able to express how your personal strengths, motivations, or activities relate to that foundation of who you are and what you expect to become.
For example, say you're in a racial minority among the kids who attend your school. You may have felt different in an uncomfortable way. What have you done to bring it all together for yourself and your classmates? How did you overcome stereotypes or stigma in a positive way?
These valuable experiences are important not only to you as a person, but also to everyone you come in contact with . . . including the admissions office who's hoping to attract honest, mature, creative, well-grounded students to make a difference on their campuses.