You may have to mix your grey between several different shades. I would experiment on a index card. Mix a drop or two of each one, and compare it after it dries. Another thing is to just go ahead and use a straight commercial color and then cover the differences with some weathering. Weathering can hide a multitude of sins.
Problem with a lot of the overseas produced stuff is that they use their own formulas for the paint and in some cases, this is not even close to the real colors on the locomotive.