If Celestial Colonnade can't attack unless you pay a cost that includes a mana payment, you may attack with it and tap it to pay for that cost
This is very different from, lets say, custom alter cards, as the art on those is the property of the people who make them, and also, you know you arent buying a real card. Play it safe if you print the back of an MTG card and put a custom-altered version on the front, and restrict its use to kitchen table rules
Youll still pay a small fortune for Beta or Unlimited, but it wont be quite as high as Alpha
The nuance of Clarabelle lies in her Shift 5 capability, which allows her to hit the board earlier than her 7-ink cost suggests
This is a bit weaker then the cards higher on this list primarily because of its mana cost
But WotC did much more than make up magic calculus to express the flavor of this blue-red dragon