It comes in three color shades. The rarest is the dark bird. Another strong flyer among the swifts. Note the width of the frontal and fairly large skull on this group as in the Anbary and the Reds. Beaks become pointed  and skinny in poor quality or mixed birds. My Mix example Otati was presented as a high quality silver necks and he is offering advice on how to avoid poor quality birds. You have many tips in that example such as the beak, color and body style.