With some dirt on the tyres and the underside of the hull, plus an antenna on the rear, I feel the Piranha PWI is finished:
I took the tyres off, drybrushed a light mud colour over the running surfaces, and spattered that same paint over the lower hull as well as a little over the sides of the tyres. The antenna is 0.3 mm spring steel that I painted OD after glueing it in place.