Welp, I finished the game with all A,B, and C endings.
Seems like there's a trick to it:
If you're aiming to get the ending A, you should make sure the sign shown at the entrance of the dice dungeon stays green.
If you want to get ending B, then turn it yellow and leave it at that.
If ending C then turn it red.
Seems like you can have a maximum of twice of the 30 years waiting for the green after starting, if you exceed then it will automatically turn into red. As for turning the green into yellow, dying 7x will turn that green into yellow.
Side note:
1. Having either Death or Waiting at 10 will trigger a game over.
2. When you finish a playthrough, you will find a kitten in the far left corner of the statue. This kitten will give you exactly how many dice you are left with from your last game.