You might want to buy extra topcoat so as not to run short in the middle of your work. I would try to match it as closely as possible to the final body color that you plan to apply. That is how Healeys came originally, so why not do it that way again. If you have extra unmixed paint left over after doing the frame and substructure, you can use up that paint later on the back side of the body panels, inside the doors, on door edges, etc. From my recent experience, many automotive paints will last for a number of years without going bad.
As others have said, automotive paints are expensive. But, as a percentage of the total restoration cost, it's not that bad.