The PSP I have seen, used as sand tracks for vehicles, is often painted in a 'local' colour. Sand brown or pink for desert use, green for Europe etc. Where camouflage was not important it was bare metal.
One paint that might be relevant for wet conditions would be anti-skid. If you are likely to be operating on a wet runway then a painted friction surface would be useful.
As for grass growing through, what is different from a grass strip? Vehicles will keep it short from crushing and you could cut the edges the same way as you would on non-psp.