The Firstborn are steadfast (I always associate them with the Roman Legions), but they are also a young and hopeful kingdom just testing its strength. I chose a blue and green scheme to show these qualities, with the tougher and sterner units (like the Iron Guard) tending more to blue, and the more impetuous units (like the Knights) having greenish cloaks.

Firstborn Mounted Knights and their Standard.

Firstborn Foot Knights and Standard. Note that I drew only the inside of their cloaks with the Firstborn hallmark checkerboard pattern – this is what I did to my Knights when I still painted miniatures. It saves a lot on paintwork and still has them sporting the characteristic checkers.
Those are swordsmen (as you might have noticed). I might one day draw alternative counters of macemen, so I can mix-and match. Just for the fun of it.
The standard is round and has the crenellated arch on a green field, matching the shield designs of a unit of Swordsmen.

Firstborn Crossbowmen with their standard. The intersected half-circle represents a crossbow, sort of.
Enhancements: Sanctified Iron, Nightwalkers, Iron Standard.

Leaders: Repulsar High Warden, Judge, Chronomancer, Firstborn Earl