From how I understand it, turning a crystal red doesn’t just happen like a mood ring, Sith have to turn crystals red in a ritualistic process.

And because a Sith uses a non-corrupted crystal doesn’t mean it turns off or stops working. From what I gather, when a crystal is aligned with you and how you use the Force, then a Jedi and their lightsaber act more as one. If a Sith uses a Jedi lightsaber or vice versa, it doesn’t mean it stops working. But when a Sith’s kyber crystal is attuned with the dark side, then their lightsaber is more responsive to the wielder.

The way I think of it, it is like a duelist using a sword they’re familiar and comfortable with. You adapt to the sword’s weight and feel until handling it becomes natural. Kyber crystals work in a similar way, but on a spiritual or mystical level instead of a physical one.