I was testing the Bloodline Bidding spell with 3 dragon-type creatures in the deck, but the spell always appears disabled in the hand even with full mana and creatures in the graveyard.
I think I’ve finally figured out the problem with this card. It’s always disabled (in red) in your hand when it has full mana until cards (I’m not sure if it’s just creatures or any cards) enter your graveyard, at which point it becomes active again that turn. But if you don’t use it that turn, it turns red again at the start of the next turn and can only be played again when cards or creatures enter your graveyard that turn.