Keep in mind that the gold (officially called "yellow" even though yellow implies a much brighter color to me) I used in my link isn't the orange proposed earlier - it's the same gold currently used for the B Roads in Australia and Tier 5 systems everywhere else (yes, even the US and Canada). So even if you go with gray (which I still don't like... it looks good when zoomed in close, but not when zoomed further out), it would still be in use in the US for a system that isn't a top tier unless a new color for the state/provincial secondary systems was also proposed.
Personally, I think using either the gold or light salmon would be a much less jarring switch than the gray. So if we don't want the gold, then I guess my third choice would be to switch to light salmon to match Europe. I don't like how the gray crowds out the other colors when zoomed out to view a whole region or how it looks drab when looking at an area that doesn't have many other route colors (such as zooming in close on Long Island or around Rochester).
One interesting thing I noticed: when using custom colors in Mapview, one can put in the color names instead of RGB values, but some of them (notably brown, "magenta", and "yellow") look different than they do when not setting manual colors.
perhaps also a revisit of how things are handled elsewhere in the world (especially as Europe and Australia are basically opposites for gold/light salmon usage).
I think Europe is fine. No need to change anything.
Everything is so inconsistent, though. It's understandable with the E roads, but especially if the regions ended up with the same Tier 4 and 5 colors, just swapped, that would be weird.
IMO the gold stands out more than the light salmon, which makes it weird that it's for the lower tier system, though Europe Tier 5 systems are so dense (to the point where I wonder if they're the equivalent of county routes) that they'll overwhelm just about any color used.
IMO just about any color scheme we can come up with doesn't work well against the OSM background; it's just the nature of the beast. When I look at mapview I'll usually change to ESRI WorldTopo; I find it easier on the eyes.
That does match up with TM quite well. Even the unclinched light salmon stands out!