What does bleeding a crystal mean anyway?
It means that a Sith takes a living Kyber crystal that has already bonded with a Jedi and forces his will onto it, and “bleeding” it in the process, turning it red. The idea is that all Sith must earn their lightsabers by killing a Jedi and stealing their crystal.
Meh.
If there needs to be a specific reason why Sith use red lightsabers and Jedi use blue/green lightsabers, why can’t it just be tradition? Perhaps in their early past, both orders used a wider variety of lightsaber colours, but they gradually stopped using such colours once they became associated with their trademark hues. Why must everything be so damn convoluted?