Your Weapon's Color or Rarity will stay the same color as your Hero's Rarity level.
You may notice that when selecting a Weapon and viewing your Hero, the Weapon's Rarity color changes. This is because the Weapon matches the rarity of your Hero's level. For example, the Blade of Justice may appear to be blue inside the Troops > Weapon tab, when selecting it and viewing my Hero it will appear Yellow as my Hero is above level 250. If you Hero is above level 500 it will show with the Mythic rarity color.