That’s why they should have all the colors.
If the Mythic Version of the helm I used as an example had all four difficulty colors, I could simply pick my favorite. You could pick your favorite. Everybody would win.
But because the Fel Green color is locked in the inferior Heroic appearance, and the Mythic appearance is stuck with purple, I have to pass it despite loving the concept of the item.
It would stick out like a sore thumb if my head glowed purple while the rest of my body and all my spells glowed bright green.