Here are some recently finished Royal Arthedain Army Knights (Sind. Reqain). These are taken from the old Harlequin/Black Tree Legions of the Realm (cute, eh?) line. They originally had the White Tree on their surcoats and all wielded swords. Lances were had from the Old Glory equipment line.
I went with a lighter blue to give them a bit of flash and dash. Silver buckles and trumpet (did you know that in JRRT's mythology, silver is a pure metal, while gold is considered corrupt?) and plain leather belts. The saddle blankets are dark blue with a silver fringe.
There have been some interesting debates on how feudal the Realm of Arnor/Arthedain should be portrayed. I tend to go with the Greco-Roman Ancient Empires model that the Prof seems to espouse - Gondor as Byzantium, Arnor as Rome. Broadly speaking, these empires had a manorial system that would survive into the Feudal period, so the idea of "lords" existing in Arnor and Gondor makes perfect sense, without the Post-Hastings culture that JRRT openly disliked (the Prof was a self-identified Anglo-Saxon who loved Beowulf, and considered the Normans about as bad as Ghengis Khan).
To bring this post back on topic, I consider the Requain on a par with the Roman Equites - mounted gentry with a military obligation. The better families who ranked below patrician were obliged to field a certain number of horsemen. Over the years of the Republic, the wars took their toll, and more and more of the better "common" families were required to provide cavalry for the legions.
Something similar is easily envisaged for Arnor/Arthedain. The manorial equestrian class supports the King's knights and draws more heavily on the better sort of commoners as the wars with Angmar wear on.
