Rainbows are formed from tiny indistinguishable dots of light and each one is produced by a water droplet from which an observer manages to catch a glimpse of an image of the Sun.
It is the precise position of each individual raindrop in the sky that determines:
Whether or not it is in the range of possible positions that will enable it to reflect an image of the Sun towards the observer.
The exact spectral colour that it will produce at any moment and over the passage of time as it falls.
The precise position of each raindrop changes over time as it falls, causing its colour to change from red through to violet. Prior to reflecting red, each raindrop is invisible to an observer. After reflecting violet the amount of light reflected by each raindrop drops off sharply.
Raindrops reflect and refract the greatest concentration of photons towards an observer from the intense bands of colour within the arcs of a rainbow.
Raindrops inside the coloured arcs, in the area between the anti-solar point and the inside edge of the violet bow, direct light towards an observer causing this area to appear lighter or brighter than the rest of the sky. Factors that determine the appearance of this area include:
Lower intensity: Each raindrop reflects far fewer photons in the direction of an observer once they have fallen below the violet band of a rainbow.
Reduced saturation: The saturation of each rainbow colour reduces sharply as raindrops leave the violet band because they mix with other droplets that are reflecting other colours.
Any situation where an observer is exposed to a mixture of a wide range of wavelengths in similar proportions produces the impression of white rather than a specific colour.
Scattering: Light reflected by a raindrop in the direction of an observer may encounter a series of other raindrops on its journey causing random scattering of light in other directions.