what string color would look best with a yellow atmosphere?
yellow
Or green, or orange.
What they said^ lol. You might also consider some of G-string’s stuff. A lot of the colours are mixed so that might be pretty cool as well. 
blue would look cool
OR:
get what YOU want 
i’m just looking for advice…g-strings are a little bit expensive
make your own string
IMO, Purple would look so sick.