The boots are silver, the path is gold and the city is emerald. Silver. Gold. Emerald. I think it referred to gems and that is how I have always interpreted the tale. I don't think I have paid much attention to the movie, but I sure did love the story. My grandma used to read it to me a lot.

So, going back to the topic - to have the wonderland it must be spectacular, hence, the gems, like silver, gold and emerald. There could be another theory - to spark interest in kids, you add colours and infuse the imagination and paint everything in bright beautiful colours. How about that?!