Excellent detail. Apart from dirt, another detail which you might like is that particularly in the 1950s / 60s, and more so on UTA than CIE, when wagons were repaired they would put in new planks which could variously be newly painted (unlike the rest of the wagon in filthy and faded paint!), or painted a different colour (maybe off a similar scrapped wagon), or not painted at all.
In the 1970s one might see standard CIE "H" vans still in grey but with one or more brown doors, or vice versa. I saw one in Ballina in brown with one grey door, one brown.
The UTA often just put a quick wash of cheap grey paint over GNR, NCC or BCDR liveried wagons it inherited. Inevitably this faded and the original colour, lettering and numerals would very clearly show through. At least one ex-GNR brake van on the UTA just ha a blob of fresh grey paint painted over the large "G" and "N"; another blob covered the GNR number, and had "U T" and their number painted onto it. The rest of the van was untouched. Ironwork was rusting and the GNR grey paint had just become a nondescript shade of greyish dirty shade, with more weathering than all the weatherers on IRM could throw at it!
I can't help feeling that black and white pictures of wagons in this state, in which the rusting ironwork looked darker, gave rise to the assumption when looking at old pics that the ironwork is black. For a modeller, think seriously heavy rusting on metal which was originally painted the same grey as a body.