I am not sure which part of your eyes change first, and I doubt that it really matters.

Spice consumption is popular among everyone that can afford it. It enhances mental powers among the Bene Gesserit and the Trade Guild because of their training. Among normal people it acts to extend lifespan and health.

Because of the cost, it is restricted to a small portion of the populace, but a very powerful part of the populace. Many among that group use color contacts to conceal their spice addiction.