The coloured rim of the primary bow is produced by raindrops near the surface of a cone extending from the eye towards the antisolar point. The cone half angle is ~42°.
Drops near the surface refract light towards the eye. At right, a drop is refracting red light to contribute to the bow’s outer red rim. It also refracts blue and other colours but these miss the eye. Drops slightly deeper in the cone refract bright blue to the eye and fainter hues of all longer wavelength colours – the eye sees a blue inner edge.
Drops even deeper in the cone produce a white inner disk for rather complicated reasons.