Colored circle around note

[Luca Rossetto Casel]

Hello everyone,

I'm working on an example for a musicological paper. I was requested to 
highlight some notes drawing a colored circle around them. I found the 
snippet

https://lsr.di.unimi.it/LSR/Item?id=722

and I tried to adapt it to my needs... but without success (I didn't 
ever studied Scheme, but I'd like to learn how to use it: all I was able 
to do is to change the color of the circle *and* the notehead).


How can I change the color of the circle, leaving the notehead black?

Thanks in advance!!!

Luca