That's a good question. Common sense would make me say that yellow bike would be more visible than a black one however, considering the condition on which people will see you, I'm not sure it would make any difference. By conditions I'm talking about people looking into their rear view mirror, not much color can be perceived, your headlight will be what they see. At a traffic light or a stop signs, they should have the time to see you, color shouldn't matter. On the highway, assume that no one can see you.