In 'The Wonderful Wizard of Oz', Dorothy's slippers are silver. They are magical shoes originally owned by the Wicked Witch of the East and given to Dorothy after her house lands on the Witch.
The shoes are important to the plot because they can carry their wearer anywhere. Dorothy does not learn their full power until Glinda explains it near the end of the story. Dorothy then taps the heels together and wishes to return to Kansas, although the shoes fall off and are lost during the journey.
The most common mix-up comes from the 1939 film, in which Dorothy wears famous ruby slippers. The color was changed from silver to ruby so the shoes would stand out vividly in the film's Technicolor process. Thus, silver is correct for Baum's original book, while ruby is correct for the classic movie adaptation.