Very Interesting theory.
A typical flame will burn yellow-orange with a little bit of blue near the base of the wick. When you burned the skewer tip coated with sodium chloride, you should have seen that the flame was pure yellow-orange (without any blue). This is because when the metal sodium is burned, it makes intense yellow-orange light. When you burned the skewer tip coated with copper sulfate, you should have seen that the flame gained blue-green traces. This is because when the metal copper is burned, it makes bluish-green light.
https://www.scientificamerican.com/article/sizzling-science-exploring-the-chemistry-of-fireworks/#:~:text=A%20typical%20flame%20will%20burn,the%20base%20of%20the%20wick.&text=This%20is%20because%20when%20the%20metal%20copper%20is%20burned%2C%20it,can%20be%20released%20as%20light.