In Rogue One there is a scene when Empire is plundering the Temple of the Kyber on Jedha.
From Rogue One dialogue:
"What are they bringing in?"
"It's what are they taking out. Kyber crystal... all they can get."
Sith use synthetic crystals so their lightsabers are red.
But natural crystal has no color until the Jedi find it.
Given that Death Star uses Kyber crystals, what explains its beam color?