I'm pretty sure it's random. I know I've gotten gear colors on Y'kayah that I never saw on Navarre, for instance. It's possible those were cases where I selected a different reward item, but as they're both primary DoW classes, I was usually selecting similar gear on most early quests, at least up until tank gear started being differentiated from other DoW. (Navarre is primarily GLD. Y'kayah is primarily ARC.)

At least the quest windows will now show you what color you're going to get on the little dot in the corner of the icon. It used to be that we wouldn't know what color it would be until we got the gear.