The color of the Yamanote Line trains is a specific shade of green called uguisu-iro (ウグイス色) or Japanese Bush Warbler Green.
The uguisu (Japanese bush warbler) is a small bird with a soft greenish-brown color, and the Yamanote Line's green was inspired by that bird. In everyday language, people just call it "Yamanote green" too.
However, it wasn't this color from the beginning. This shade of green was first used on the trains that were gradually introduced from 1963. Before that, the Yamanote trains were canary yellow - the same color currently used for the Sobu Line.
Here’s the story of the Yamanote Line’s various color permutations:
