What color were Dorothy’s ruby slippers originally in “The Wizard of Oz” books?

Everyone knows the dazzling ruby slippers from "The Wizard of Oz." Nearly 90 years later, it's practically a celebrity in its own right. The shoes that journeyed down the yellow brick road have spawned countless replicas, Halloween costumes... and even museum heists. But ruby isn't what author L. Frank Baum had in mind. What color were the slippers that carried Dorothy home in the original book?