The original color of the gas tank was silver/grey. I spray painted mine when I had it out for the fuel pump. With rustolium silver.
The POR-15 is great stuff however. I did my transmission with the Sterling Silver shade. Looks great.
The nice thing about silver gas tank is that you can see it from the car behind you. Black suspension, yellow shocks, silver tank. Nice combo.
But if originality isn't a priority, black would be nice, tank would blend in unnoticed.
Your car...your choice.