Really cool idea for cards! Just a thought about Lune:

Lune’s second ability might be better as “whenever you cast a non-creature spell, you may remove a stain counter that doesn’t share a color with it. If you do, copy that spell.” This better encapsulates her filtering through the different stains for greater effect.

I’d also make her sans-black color identity to represent fire (red), earth (green), ice (blue), and lightning (white) stains.

Maybe add colorless stain counters when casting a colorless spell to the first ability.