It looks as though the colour in Greek is emerald rather than a generic green or blue.

σμαραγδίνῳ smaragdinō - https://biblehub.com/greek/4664.htm smaragdinos: made of emerald, emerald-green from σμάραγδος smaragdos: an emerald.

https://biblehub.com/lexicon/revelation/4-3.htm