Oh, God of dust and rainbows, help us see
That without dust the rainbow would not be.
by Langston Hughes
I'm a fan of Hughes' poems but this just seems completely confused. Rainbows are not effected by dust but rather water and the general diffusion of the atmosphere. You could have a dustless world with rainbows. There was once the belief that dust and water vapor in the air made the sky blue, although why it didn't change color during dry heats and wet heats never seemed to get addressed. It is actually because of the diffusion of the atmosphere itself (photons get pushed a bit by the EM of the atoms).