This might be getting into more than we want right now, but instead of arbitrarily-numbered tiers, perhaps it makes more sense to have named categories that make sense in each part of the world along with a partial ordering that defines how they get prioritized on maps and in lists.
That's what I've been thinking, but not come up with a fully fleshed proposal. Freeway, Continental, State, Local, Tourist with a <name>2 sublayer for each (might as well give them all one, even if only some are used) seems sensible, albeit with the names I've used being rather US-centric.
It also allows the hiding of certain types of routes (eg tourist routes) to be easier too.
At the time I checked it out (2017, 2018?) there was a clear color hierarchy that held site-wide.
That's no longer true.
I'm not sure that was ever true. At least not after E Roads were added as a tier 3 green system in 2009 or whatever.
Lightsalmon in Australia is mostly a place holder system - it's used on the NT Territorial Highways (which is being converted to alphanumeric) to separate them from the C roads (which are grey), and the Melbourne Metropolitan Routes (vs Victoria C roads) too. I can change the colour to grey, but I've just not bothered doing it - the lightsalmon meaning different things in different parts of the world is less of an issue, IMV, than grey meaning different things within the same region.