grey or white over the top will look like... a grey or white overspray...
Try a lightened with tan version of each colour, but in the centre of each individual panel.
Spits were cosseted & looked after, except for Africa/far east, they didn't generally get that scruffy...
then try a pro-moddeler or 'sludge' wash for dirtying it up a bit, to accentuate the panel lines.
It's your plane so do what seems right to you.