There has been some discussion about what might be a similar problem on talk-us, where different states have different signs on Interstate highways.
I have a few questions; it might be good to explain the answers on the tag proposal page. (I'm sure in .es the answers are obvious but I think road sign practices are quite different in the US so I don't understand.) Is the sign color part of a national/regional standard scheme? Typically, are all roads of the same administrative class having the same color signs? Or do roads of the same class have different color signs so people can follow the blue road or the yellow road in case reading numbers is confusing? Or does it tell you a road subclass - like the "no roundabouts"? In the US, the essence of the issue is that we have Interstate highways, which have a standard sign, but some states, especially California, have variants, and people want maps to show the local variants so they match what's on the ground. The proposal is to tag the road as being an interstate in California so the renderer can find the interstate/california sign variant. Then, there are US highways, which have similar signs most places. With state highways, each state has their own sign and color scheme. But I'm not aware of varying colors other than as part of a class of road. (I'm of course not trying to say your colors don't matter - just trying to tell you about our signs, so that any tagging schemes might be as general as they need to be.) Using ref:color means that has to match ref. So there's the question of when a physical stretch of road has two route numbers. (This happens often in the US and I would think it must be common in Europe as it's hard to avoid without having people not to be able to follow the lesser road.) It seems for that one needs to move to relations with no ref tags on ways, but only on the relation, and then the way can be in two route relations. So with that ref:color applying to ref works, but there has to be a "one ref tag" rule. Once you have "ref=A19;C34" the ref:color seems unworkable. int_ref vs ref seems funny to me. E-5 is just another administrative classification, and the fact that it crosses country boundaries doesn't seem that much more important than say US highways crossing state boundaries. One of course needs to know the network and number, but a different tag feels denormalized (in the database schema sense). The US discussion on sign rendering seems to be headed towards "network=us_i" and "ref=95" for Interstate 95 (I-95). I guess my biggest question is if the colors are arbitrary or they are encoding some property of the road. If they are telling people something about the road, then perhaps those rules are what should be encoded and then renderers should have access to the (jurisdiction,property)=>color table.
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