Maybe start with darker shades of grey for the vehicles?
But generally camo gets brighter ( thanks to the sun ) on vehicles and repainted patches are often darker than the bleached "original"..
at real Tanks that is. Sure in
40k , paints could have some secret ingridients to prevent that.
IMO, personal gear of the soldiers is sooner replaced than any part of a vehicle and the camo should be darker on your infantrymen
( which sit in that chimera and thus in the dark..) than those of the vehicles.
PS: as part of the "factory fresh" group, my Tanks are always not-weathered. The Guard does a lot of things, mine got the honor to keep their vehicles in good condition and until now the Munitorum didin't complain about the requested amounts of paint....