So here with Blue, Black (typically the colour of stealing life or outright destroying it) and Red, you will see creatures such as Cybermen, Daleks and many more besides
Imagine if every time you made a new Ooze you got a free cast of In Garruk's Wake
Anytime you attack you get to exile the top card of your deck until you hit a nonland card, putting three time counters on it and suspending it outside the game
We tend to send him in early even into open mana, because the card advantage matters more than the four points of damage
However, for decks that dont revolve around the mechanic but still have a couple of Day and Night cards splashed in, it can provide an additional unneeded layer of complexity and mental load to the game
Here are the details of Hearthstones system with Magics rarity equivalent for comparison: Its a pretty straightforward concept