Higher Learning's Tyra Banks says color barriers definitely exist, even when you're a supermodel-turned-actress. ''Every now and then when I walk into a department store I won't be helped,'' says Banks. ''You know who's really bad? Saks (Fifth Avenue) and that's my biggest (modeling) client! (I say), 'You don't understand, you guys are payin' me up the roof.''' Saks has no comment...Winner of the Most Auspicious Debut on a hot television series goes to ER's new arrival, Ming-Na Wen (Street Fighter), who plays young Dr. Deborah Chen. Her character's first doctorly duty was to perform an unlubricated rectal exam on an ailing patient. (''To really get into the character, I brought my own latex gloves,'' she says.) The downside of such a beginning? Says Wen: ''The (rectal exam) is what I'm going to be famous for.''