To answer your first question, just place the flies on a paper towel until they are dry: no rust or rot. Consider that every fly that you have fished gets wet. You put it on a drying patch after using it. Then you put it back in your fly box after it is dry. If wetting flies caused rust or rot, flies would be a one use only product.
To answer your second question, most of the time fish are feeding opportunistically. So the great majority of fish would not be selective to the point of refusing a fly that does not match the coloration differential of the anterior and posterior of the natural.
Yes, a dual color fly is a better match than and single color fly, but that is an advantage ONLY WHEN the fish are selective enough to refuse the single color fly and take the dual color fly. The question then becomes, how often will that occur?