Good eye! The swing trail was picking its colour from your character's level instead of the weapon you're holding. It now follows the weapon, so a high-level weapon shows its proper trail colour. Fixed in today's update.

The color of the swing should be determined by your weapons mastery skill, not the level of the weapon. If I have max sword mastery, I should have an orange swing trail even with a level 0 sword. But if I switch to a spear, it should be blue no matter what level the spear is because I don't have a spear mastery skill.