Does anyone know if there are banjo heads out there that are top frosted, but instead of white, are custom painted to my requirements (it may be colours of my country's flag or a mountainous scene or a sailboat scene or whatever)...without changing the "sound" of the banjo head?
Yes. There is a fellow BHO member out there who who was doing this service a while back...putting flag designs on banjo heads. But what he did was paint the underside of a clear head. Any design applied to the top of a head will eventually get spots worn through just from playing. And if you paint the underside of a frosted head, some (maybe a lot) of the detail of your design might be lost through the frosting of the head. Good luck.