I just watched this lecture on rainbows https://www.youtube.com/watch?v=iKUSWJWMSk4 and in it I found out that the colors in the rainbow are not monochromatic. At any point in a rainbow there is a peak light frequency, but all the lower frequencies are also present. So the red side of the rainbow is nearly monochromatic but the blue side is mixed with white light. My question is how many times more intense is the blue light than the white light it is mixed with?
The reason I want to know this is because I want to know which visible colors can't be made by mixing colors of the rainbow. If the blue light is say 10 times stronger than the light it is mixed with, then there's probably only a tiny sliver of visible colors that can't be made from a rainbow.
Another reason why the colors aren't perfectly monochromatic is that the intensity peaks aren't perfectly sharp, but I'm guessing this is a less significant factor.