Why are the primary colors red, yellow and blue?

Light spectrum is continuous. To reduce the cost of information processing, humans only have three types of light receptors. Each type is most sensitive to light with specific wavelength. Together, the three colors these light receptors are most sensitive to are called primary colors. The visible spectrum of light stretches from red to purple. The main colors of a rainbow are red, orange, yellow, green, indigo, blue and purple. These are the seven main colors. From the list of seven colors, you might feel red, green, the fourth and middle color, and blue are good candidates for the primary colors. Why are red, yellow and blue color the primary colors for humans?

The full spectrum of lights might be from red to purple. But at dawn and at dusk, the high frequency lights are mostly filtered away by the atmosphere. The leftover colors are predominantly red and yellow. Overall, the most efficient choice of the primary colors is red, yellow and blue. The primary colors of humans are evolved to process visual information most efficiently at dawn or dusk. That is why photos taken at dawn or dusk are often the most beautiful.

In early days of color TV and computer display, the primary colors were red, yellow and blue. Today, the primary colors are chosen to be Red, Green and Blue. This is RGB scale. It is indeed more efficient at the technical level.