Hello!, when i had to change the calendar colors i do the next:

See with f12 in the navigator from where come the "colormap" (all the colors)  in my case are "full_callendar.css" (always gonna be a .css or .scss file) for general views can be assets_backend.css , 

the after have to search where defines something like "colormap" or similar, and gonna find some formula where you can edit for define the diferent colors