Flame Test

Electron continuously jumps up to a higher energy level and comes back. Electrons keep jumping to and fro continuously, that's why you continuously see the colour. If it would have jumped once, then only one ray would have been emitted.

You cannot see two colours because the energy from the flame makes electrons move up to a fixed energy state. If you could make the electrons move to some higher energy state, may be some other form of electromagnetic wave (or some other colour) would be emitted. But, you cannot do it as it requires tremendous amount of energy.

Hope you understand...

What if I continue heating the metal?

No. Even if you continue heating the metal, you will never be able to give such a high amount of energy so as to make the electrons jump up and down to and from a higher energy state.

Thanks.