I’d certainly love for a lot more stuff to be tintable.
That said, I think that in the case of metal blocks/metal props and the like though, as far as I have seen the technical issue would be that they already use the tint system in a way, but there are basically only a tiny amount of tints, and they can’t be changed apart from how they were crafted, such as with metal signs. They basically have a range of 0-4 (copper, iron, silver, gold, titanium).
The normal tints for blocks range from 0-255 (an 8-bit Byte, I think, from black to luminous yellow, iirc), which I would think is the limit of the range of tints that can be reserved for a block, so I’m guessing that to allow things like metal props to be tintable with normal colours, the amount of data reserved to tints overall would have to be wider, so that metal props could benefit from the full range + have their own unique metal tints, and if they were only going to add 5 more tints to the system (effectively), it would leave a lot of spare reserved tint data that likely wouldn’t get used.
The technical solution I can think of, that may be no better in terms of efficiency, would be that when a metal object is sprayed, what could happen “under the hood”, is that it could become a special variant such as, let’s say: ORNATE_METAL_POLE_TINTABLE instead of an ORNATE_METAL_POLE, which would then have a metal specular map or whatever + a mask of some kind that tints that reflective/metallic layer. This solution would require no changes to the amount of data that gets reserved for tints for any block, but would effectively create a large amount of duplicate items…
The only reason I’d suggest this as a technical alternative, if it’s because of the limitation I assumed of a 1 Byte tint data, is because there’s already a number of _DUGUP variants of a few blocks/props, so it doesn’t seem too far-fetched to me to simply have tintable duplicates, as a solution.
But I’m making a lot of assumptions on how things are working, and it’d be best if a dev (like @james) could clarify on what is and isn’t possible.