Ah, I didn't realise that iD suggested different tagging for trunk and primary (although historically people have misinterpreted iD's "Not Specified" as "I need to set a value here").
The UK tagging of "green=trunk; white=primary" is widely accepted (see the reaction to https://lists.openstreetmap.org/pipermail/talk-gb/2021-September/thread.html#27754 , also mentioned at https://wiki.openstreetmap.org/wiki/Tag:highway%3Dtrunk , https://wiki.openstreetmap.org/wiki/Roads_in_the_United_Kingdom ) isn't especially "odd", even though it doesn't match what some authorities might classify roads as - it's an easy to verify rule.