The current module constrains to values in enum but khal supports RGB
colors as well
khal.readthedocs.io/en/latest/configure.html#the-calendars-section !
(be careful when setting an RGB value, it has to be quoted else it is ignored, got bitten by it with a manual config )
NB: It's also not possible to set addresses khal.readthedocs.io/en/latest/configure.html#the-calendars-section