(Note: I found another suggestion similar to this, but I think my idea is different enough to be its own suggestion)
Whenever a card with a seal is interacted with, there is a 1/3 chance to treat it as a random seal which is relevant to the interaction.
Explanation:
Whenever you discard a card with a (non-purple) seal, it has a 1 in 3 chance to act as a purple seal since the purple seal is the only one which interacts with discards.
Any steel card with a seal which is triggered can act like it has a red seal.
Played cards have a chance to choose between red and gold seals.
Cards with seals held in hand at the end of round have a chance to be treated as blue seals. Unless they're gold cards in which case it chooses between red and blue seal.
Of course the numbers can be tweaked and simplifications are welcome. Maybe the idea as it is currently isn't very vanilla friendly but I feel like some form of this could exist in vanilla.