If you can get some of the white reflective stuff they use on stopsigns, consider painting the tank a semigloss black, then put on a reflective design with the stopsign stuff and lay a couple coats of glossy black clearcoat over it.
The effect is amazing, especially at night. When someone sees you riding along, they see an all black bike. Then the light catches the design just right and it flashes at them, then the bike goes all black again snd they're never sure just what they saw.
