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.
The second Death Star fired on several Rebel ships during the battle of Endor, and the beam did not immediately turn red due to the horrors of war. This process, therefore, must presumably take several shots at minimum.
(Has anyone checked to see if the Death Star shots in Rogue One are less red, on net, than the one in A New Hope?)
the original death sar fired on a city in rogue one, i remember that much. or whatever you'd call it, it was a populated area
but i imagine it's hard to do a "less red" green than the green of a new hope bein near 00FF00

