How can you make tan paint darker?

You would have to add a darker color, or take it to a paint store and have them add colorant. Trying to change paint from its original mixed formula is not a good idea. All paint brands require different formula's for the same colors. So if you were to try and change a color from its original formula by adding a different color of paint, it would be impossible to match the new color if you were to run out before finishing.