I have an indirect system with an elderly gas boiler.
Just recently I've noticed that the first cup full of hot water in the kitchen is grey colour, not noticed this with the bathroom hot tap. My thoughts are the coil in the hot water tank is leaking heating circuit water into the tank. But why would this not show upstairs? Could it be something else?

Jim

Your idea could be correct but usually the boiler water is at a much lower pressure so actually the flow would be from the house water line to the boiler coil. Also it could be something to do with the pipes that service the kitchen water valve.