Changing Key Colors
Keys (or Frets) colors are stored in the fretsonfire.ini under the [theme] section (as explained in the forum's Modding FAQ)
[theme]
fret0_color = #22FF22
fret1_color = #FF2222
fret2_color = #FFFF22
fret3_color = #3333FF
fret4_color = #FF22FF
These are the default values but it is possible to change any of the HEX color codes. For example, to get an orange last fret instead of a purple one, fret4_color could be changed to FF4400 or FF6600.