There's a lot of colors that aren't represented that could be, heck they could make all warm colors attacker colors and all cool colors defense colors then let you pick your own palette, but doing that would give them one less thing to unlock so it's highly unlikely they'd give that sort of freedom even if it'd likely be well received. I mean why should you be limited to having to have two colors? Some pieces of equipment let you remove the paint, but not all, it makes sense that the standards should be colored, but you should be able to pick a solid color without having to unlock one of the very limited skins that have two of the same color stacked so it becomes solid.
Instead of asking for a specific color scheme why not try to make everybody happy and ask for them to be replaced with letting players pick their own color and just limit the colors to the appropriate colors based on them being neutral, attacker, or defender?
Neutral Primary - Any Color
Attacker Primary - Warm Color
Defender Primary - Cool Color
Neutral Secondary - Any Color
Attacker Secondary - Warm Color
Defender Secondary - Cool Color
Neutral Tertiary - Any Color
Attacker Tertiary - Any Color
Defender Tertiary - Any Color
The third color is usually a neutral color like white or black, but since it's just detailing on some patterns and the emblems it should be able to be any color as long as the primary and secondary clearly display that the person is either the attacker or the defender in those modes, but to be honest I don't really see the point of colors to begin with when they could just give them some sort of identifying ring around their feet I mean it's pretty clear who you need to fight when they have the identifying attacker or defender logo floating above their heads that should be obvious enough, but meh everybody would be happy if they atleast got to pick what color in the appropriate spectrum (attacker or defender) or at least happier. I mean the Warden can get gold armor and if they're trying to say colors are that important that they have to be used as identifiers then why is that a thing? The gold can easily be more prominent than the colors on the standards.
They could at least give us a solid pattern so I don't have to have some odd design that doesn't line up with the armor on my vikings and samurai, the only ones the patterns honestly work on are the knights imo where you can disable it on the head and arms and just have it on the actual cloth that's designed for that sort of thing.
In regards to there not being a forest green I'd wager it's the same reason there isn't a brown and there's only one solid dark blue option and that's because from a distance the lower vibrance colors like forest green could appear black from a distance and considering brown is a shade of orange and forest green would be a shade of green you could start getting them mixed up as to who's the attacker and defender if you are one of the people that takes the time to give the other character that's trying to punch you in the face the up down just to make sure it's the opposing player, but to be honest, there is so much friendly fire going on you might as well consider everybody that isn't in your group hostile what with everybody so desperate to get credit for that kill that they'll attack you and knock you out of attacks just to get their swing in even if it gets you killed in the process. (They're also the same ones that cry first when they walk into an attack you've already started and it kills them)