According to Christopher, if your vitamin C serum changes colour, it might be time to throw it away. "When vitamin C is exposed to air, it oxidises and changes its form into something that isn’t as effective and it doesn’t work as well on your skin," he said. "When the oxidation happens in a product, it goes from a lively golden colour to more of an orange-brown colour. That’s your visual clue that you might not be getting all of the vitamin C the product says you’re getting." If anything, Christopher says you’re probably getting more moisturisation from other ingredients built into the serum base than the actual vitamin C itself.