Dream Coat

The free activation is the design pivot here. Color-changing effects had existed before, but pricing the recolor at zero mana (with a once-per-turn governor) turns the aura from a combat trick into a permanent shapeshifter: pay one blue mana up front, then rewrite the enchanted creature's color every turn for the rest of the game at no further cost. That structure is what the card was built to enable. Protection from a color was the dominant evasion-and-defense keyword of the era, and Dream Coat is the answer for both sides of that fight: dodge the opposing White Knight by becoming the wrong color this turn, or push your attacker's damage past a protection-from-blue blocker by becoming red for the swing. The once-per-turn cap is what keeps the loop fair: it does not stop the creature from wearing every color at once (the text explicitly allows "color or colors," so a single activation can make it all five), it stops you from recoloring repeatedly within a turn to answer whatever lands on the stack. The card makes a small, elegant statement about how cheaply a repeatable activated ability can be priced when its use is throttled to once per turn, a lever Wizards would reach for again and again in later eras. As a piece of history it is a footnote from a set crowded with louder enchantments, but the rate is genuinely strange: a one-mana aura whose ongoing effect is free.