apple barrel's Dolphin Gray is a good starting base. The hue may change depending on whether you're painting a TOMY or Trackmaster running board. Like Therealblack64YT said, just lighten/darken depending on what you're matching. But Imo, unless you're doing very small touch ups, you should repaint the entire board from scratch because of exactly what leylandvictory2 has stated. Preferably with an air brush.