Colour accuracy... Lol...
My old man was a Gunner from the '60's to '80's, I've seen vehicles & SP's in the gun park & in firing positions, in various states from 'Bull-Shined' to in maintainence mode.
In all states, the shade would vary from paint-shop new to tired. (precise technical terms used there )
SP's & Guns especially would not be parked up without going through 'wash-up' first though, std RA practice.
Having said that, I do keep up with current thought, eg using the appropriate Vallejo primers (Sandgelb/OD/Bronze Green/Russian Green) as the STARTING POINT.
I also notice that there are different versions of OD, even within Tamiya,& try to start with the version I think appropriate, eg using Revel's German OlivGelb for pre-NATO vehicles, & Xtracolour British RAF colours for early British Planes...
I am experimenting with HATAKA's new set of British Armour Colours, too early yet to pontificate.
Point is, start with the appropriate pallette of colours, (preferably from the same mfr), & work to what seems good to your eye.
Plus it all depends what paint ranges you can actually get your hands on!!!