A primary rainbow is brighter than a secondary rainbow and has colors changing from red on the outside to violet on the inside.
Photograph by: Olthoff
We will focus on two raindrops to explain why this color pattern develops. Red light from the higher drop is directed toward the observer's eyes, while violet light is directed at a level above the observer.
From the lower drop, red light is directed to a level below the line of sight, while violet light is seen by the observer. This is why the colors of a primary rainbow change from red on the top of the arc to violet on the bottom.