This is how you see them in the sky:

A rainbow happens when it is raining in one part of the sky and sunny in another.

Rainbows are a phenomenon caused by refraction, light reflection, and dispersion in water droplets.

The colours of the rainbow are red, orange, yellow, green, blue, indigo and violet.

Double rainbows occur when light bounces inside the water droplet more than once.

This is how they are made at Orchard Press Cheltenham:

          Our Rainbow Software adds 3 spot colours to CMYK profiles to offer exceptional stand-out colour and quality. This undoubtedly is the highest quality printing service in the UK and is exclusive to Orchard Press!

You provide high-quality images and artwork in RGB format and we will produce vibrant, high-colour saturation, contrast, luminance and
purity that standard CMYK just cannot match.

Using seven colours allows us to produce a wider colour gamut and to reproduce images as true to the original as possible.

Contact us for one of our exclusive Orchard rainbow brochures to see the difference https://orchardpress.net/brochurerequest/