Technicolor pictures were still a new technology and a big deal when 'The Wizard of Oz' was released, and the extensive use of color in the film was deliberate, with the producers wanting to make use of the new medium to its full effect. That meant changes to Dorothy's footwear. Her ruby red slippers are just as iconic as the yellow brick road itself, but you won't find any mention of them in the original books by L. Frank Baum.
Baum, who couldn't possibly have known he should have been writing with an awareness of how things might look in a movie theater forty years later, gave Dorothy silver slippers. It was screenwriter Noel Langley who made the call to go with the dark shade of red, correctly believing they'd stand out more. Somehow, the thought of Dorothy clicking silver heels together three times to get herself home just isn't as appealing.