I guess it's technically to keep everything consistant across the board. All the hero units have a red sword but their weapon dictates ranged or melee.
I missed that Lilly has the red sword/melee symbol as well and not a green bow&arrow for ranged weapons as you would expect from an archer. So now I find it even more inconsistent than before
But, it could be that red was chosen to depict "Battle value" in general, no matter what kind of weapon. Although, I find this a less than ideal choice. As the heroes can equip whatever weapon they want, it's less of a worry and "ok".
As for the minions, I guess you'll have to figure it out once and than you'll pay attention to what kind of weapon they have. The question is just if that ONCE is after/during the first chapter or much later...