Hi,
Once the track is glued down, I first used a thin wash of Phoenix precision paints sleeper grime to take the sheen off the plastic sleepers. Then I ballasted the track with a 70/30 mix of grey and brown ballast. Once it had set, I painted the rail sides in Phoenix paints light rust, being careful not to get it on the sleepers. Once dry, I dry brushed over the tops of the rails with sleeper grime so as to leave a small deposit over the sides of the sleepers and ballast. This will also add extra colour variance to the rail sides.
Hope this helps.
Best regards,
Jeremy