I am not entirely sure how I should approach answering this question. The Emerald City in this childhood classis is described with the colour green because the emerald is a precious stone whose colour is a shade of green that is notable for its brightness. Often it can have a slightly bluish tinge, however the overwhelming impression is that of green. So, just imagine you were going to visit a city that is entirely made out of emerald. What would strike you most of all is the cumulative colour green in everything. Thus the Emerald City is green and green is used so much to describe it because the emerald itself, as a precious stone, is green, and famous for its translucent nature. We can obviously only imagine what it would have been like to approach the Emerald City, but we can guess it would have been pretty amazing for Dorothy and friends, and very dazzling.
Emeralds are green. Or most are. It's as simple as that.