>>42382705 (OP) Red, yellow, and blue are the primary colors. They are all you need to make the rainbow. A full 6 color rainbow is too detailed for a cutie mark. The 3 colors are a simplified rainbow.
>>42382945 But a rainbow is the result of a refraction of light, not the application of paint or pigments. The three primary colors of light are red, green, and blue.
>>42382957 Yes but red, yellow, and blue are the colors you need to create a rainbow in artistic settings. In the show, rainbows aren't created from light refractions. They're created by mixing colored liquids together at the weather factory.