When burning a salt, how do we know the flame color comes from the metal ion and not the non-metal ion?I did a lab on this by burning different...

You don’t exactly burn a salt. When you heat the salt it
splits into the metallic and non-metallic ions.

Due to the heat the
metallic ion changes state and when it returns the electromagnetic radiation given off
has a particular color. For a compound, it is always the metal that is involved in
determining the color that is present, the non-metal atoms have no contribution
here.

For evidence you will find the color yellow given off when all
sodium compounds are used, all strontium compounds give red, all potassium compounds
give a purple radiation and copper compounds give a color between green and
blue.

It depends only on the metal in the
compound.