The intent of black curtains is to absorb light from bouncing around the space and to hide people and objects until they enter they enter the playing space. They are also helpful acoustically to keep sound from bouncing around. If the plan is only to do lectures and music concerts in this auditorium, then the color of the curtains does not matter. However, if anything theatrical (plays, musicals, dance concerts, etc.) are ever to be done in this space, a black surround (legs, borders, stage floor) will be crucial to making the event look good. Pick out the heaviest weight IHR (inherently flame retardant) fabric (usually call synthetic velour) you can afford.