I recently started looking at underground city in moves and animation and movies such as Arthur and the Invisible and Bug's Life where the color scheme is a bit lighter and softer as well as welcoming.
Fig.1. Bug's Life
Fig.2. Arthur and the Invisible
While doing some research for my city ARGIA i started to look at underground city and I look at the world tree theory when a whole world existed under the biggest tree.
Fig.3. World tree example
Fig.4 World tree concept (not mine)