In modern continuity, kyber crystals do not naturally produce a red blade. Instead, to forge their sabers, a Sith has to take a lightsaber from a Jedi, and through the force, break the will of the crystal in a process called "bleeding". By exposing the kyber crystal to enough horror and trauma, the Sith changes its fundamental nature, and bends it to their will.
The Death Star's laser was built using tons of harvested kyber crystals. In A New Hope, the Death Star only manages to fire a single shot of green energy, which destroys the planet of Alderaan and sends a shockwave of agony through the Force that can be felt lightyears away.
I think if the Death Star had been able to take a second shot, the color would be red.