You wouldn't be able tell if the RA was open air , British climate taphonomy etc .Newgrange is well protected and dry , no evidence of anything from there , doesnt' mean that they wern't highlighted or coloured though . Freshly marked rock is pretty striking enough on it's own with good contrast of old surface and new and if quartz is used the markings are brilliant white , for a while .Some markings may have been "re-done " .