Usually when we dream in black abd white, it's our subconscious trying to sort through things without the 'distraction' or emotion of colors. Much like an old, "Just the facts ma'am", movie.

The dream of your friend was probably brought on by a familiar place that you both visited, a photograph, or a smell...something that reminded you of her. That your other friends couldn't see her, would suggest a memory that only you and she share. It was a bittersweet walk down memory lane.

Hope this gives you some insight.