I voted Other and am advocating for adding other colors here.
Short version:
In my opinion Jaina should use whatever color you decide for Dalaran in the Human & Undead campaigns, then switch to use Theramore's color in Orc & Night Elf campaigns, then from there, she could use any of Daralan/Theramore/Kul'tiras, depending on what you think is best. However, I advise using a color that is not "#4 Purple" to avoid conflict with the Undead Scourge led by Arthas.
Long version:
I am limiting my argument to WC3 and the classic 3 of WoW (vanilla, The Burning Crusade and Wrath of the Lich King) because Jaina's personality has been consistent during those times. Also, the writings and stories there are just superior than modern WoW.
When I say superior writing, I mean that almost every character in the story is a manifestation of a mythological archetype. This is very appropriate for a high fantasy like Warcraft.
What does this have to do with Jaina's team color? Because she is (in my subjective view)
the very heroic representation of the water element she uses ---- her loyalty constantly changes and flows around, without solid boundaries in order to nurture what is the greater good for the world at large. The constant changing of her team color is a very good way to demonstrate this.
Therefore, I firmly believe that
it is very appropriate for Jaina’s character to have different colors throughout the game. She does not have a selfish agenda, but rather just want to bring about whatever she think is best for the most people. This compels her to constantly switch factions in order to help out other morally good characters in the interest of greater good.
Whatever you decide, I think it is best to use a color that is close to Purple but not Purple itself. These are the current 24 colors available to users:
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If you're using Purple for Dalaran, instead of using "#4 Purple", I advise considering "#16 Violet" or "#20 Lavender" to avoid conflict with Undead Scourge.
From a technical view, this will also allow her to encounter and face Arthas once he's become a Death Knight.
Notice how in vanilla WC3, Arthas and Jaina never met again in any of the campaign after the Human one. I find it a very wasted opportunity.
I feel maybe Blizzard's writers caught up to this. Fans do want to see more dialogues between Jaina and Arthas. They have a lot of things that need sorting out. The entire
Halls of Reflections dungeon in WoW: WotLK, where Jaina led a band of adventurers to pursue Arthas? It was not about Jaina fighting the Lich King. It was about Jaina believing that Arthas' soul is still alive somewhere in that body controlled by Nerzul, and based on that desperate belief, her doing everything she can to rescue him from the Lich King: Yelling, scolding, inquiring, pleading...whatever that will get the real him to awaken and remember her. Whatever that will let Jaina have the real Arthas back...even for just a short period of time...even if that time will be so brief she only has the opportunity of a few seconds to say one last goodbye to him before they depart from each other forever.
I would love to see such a encounter in this new campaign you're reforging.