Provide an explanation at the subatomic level to explain why different colors are observed for different metal salts.

Due to difference in the electronic energy level of atoms in different metals.

Explanation:

The subatomic particles are protons, electrons and neutrons.

When we talk about colors, the subatomic particle involved is electron.

An atom of a metal absorbs light and in this process the electrons gets exited (gain energy) and when they come back to ground state they emit light and thus the color appears.

In case of different metals the energy required for excitation of electrons is different (due to difference in the arrangement of electrons in the orbitals and difference in effective nuclear charge), thus the electrons in different metal atoms absorb light of different of wavelengths and end up in appearance of different colors.

When you heat an atom, some of its electrons are "excited * to higher energy levels. When an electron drops from one level to a lower energy level, it emits a quantum of energy ... The different mix of energy differences for each atom produces different colours. Each metal gives a characteristic flame emission spectrum.