You can mix the grey paint you used with white to get it little bit lighter and then use it on the upper sides of the hull and turret. Then make it just a little bit lighter for the top of the hull and top of the turret. Leave the very lower sides of hull as is. This would simulate fading causeed by the sun and then you can use a dust wash to get the effect you want. MIG, Vallejo, and AK all make a good dust wash.