The CVR stuff certainly appears to be a mix. CIE narrow gauge wagons ended up in a deplorable state - thus, rust.
A few CDRJC wagons appear to have had some black ironwork, but others all grey.
When jhb171Senior visited the CVR in 1937, nothing black was to be seen on any wagon ironwork - at least, not by him. However, I have certainly seen photos showing black on some stock at least, so it is likely that the policy changed at some stage.
As Mayner says, however, on the NCC, UTA, GNR, GSR and CIE, ironwork was never picked out in black. Nor on the BCDR. Exception: the BCDR had eight private owner wagons belonging to C Ritchie. Initially at least, these had black ironwork. The East Downshire wagons also look as if they may have had black ironwork, but photographs are inconclusive given weathering and rust.
In reality, black and white photos are highly unreliable for colour shades - very obviously!!