I've never used coffee. Just tea.
Coffee and tea couldn't possibly stain the hair much at all and if there is a bit of stain, would you think it would last?

Has anyone been using coffee or tea on their own to colour hair?

The reason henna stains hair is that the pigment binds with your hair protein. It becomes part of the hair. Coffee and tea do not do this- otherwise wouldn't we be using it to colour hair already?
It will be very bright orange at first on white hair. Then it oxidizes and gets darker over a day or two.

If you want brown and want to stick to natural products you have to use indigo with your henna.