"Kyber crystals were inherently attuned to the light side of the Force, and would attempt to resist any effort by dark side practitioners to use them in lightsabers. To this end, the only way a Sith, or other darksider could make use of a kyber crystal was to use the Force to dominate the crystal, bending it to their will. This process would cause the crystal to "bleed", as if it were a living organism, resulting in the distinctive crimson-bladed lightsabers synonymous with the Sith. It was possible to "heal" a corrupted kyber crystal, the only known instance of purifying a corrupted kyber crystal was when Ahsoka Tano purified the two crystals she retrieved from the Sixth Brother's lightsaber following his defeat. In this case, the crystals turned white." - Wookieepedia
Honestly, I despise the new explanation. I think it's stupid that Jedi can now color crystals themselves. It takes away the authenticity and beauty of crystals, at least for me. When I was a kid I thought Ilum was so cool, and I envied how the Jedi would meditate in the crystal caves and the crystal would essentially choose them. As far as I'm concerned, natural crystals start off in balance with the force and with the color that they grew with and red crystals are/were created synthetically by the Sith. What are your thoughts?