Fretboard note

More experimenting. The game/quiz I'm building consists of 3 types of question:
1. Given a string and a fret, you have to choose the correct note.
2. Given a string and a note, you have to choose the correct fret.
3. Given a fret and a note, you have to choose the correct string.

I'd like to associate the three question types (note, string & fret) through colour. In this example, a note would be red. Also trying to use icons to reinforce that association. The only problem...how do I represent a FRET using an icon? :)