Depends on what color your fuel tank is.... Black or dark colored fuel tanks are easy. I use some high gloss paint and paint the spill down the filler pipe and tank AFTER the engine has been weathered. The fuel is oily so it looks glossy and the longer it sits on the tank, the more the oil builds up with dust and grime. You can also dab some weathering powder in the high gloss before it dries to look like the oil has collected some dirt and grime.
Light color tanks I use black paint, dab my thin brush in the paint, and then fill the paintbrush with thinner. Then dab it on the tank and filler. Get just enough black on there to notice and the add thinner to make it run as desired. If you don't like it, just use a bruch full of thinner and wipe it off to start over. Just remember to have clear coted your weathering or the thinner will eat off the weathering job.
Don't forget to use the same process on modern locomitves where the pop off valve is at the end of the main resivor. There is always a build up there fom the wet air being blown out onto the top of the fuel tank.