I'm a bit of a Star Wars nerd, and aside from the obvious symbolism of the Dark Side of the force having red light sabers, I kind of always wondered why they were always red. What if they stole their saber originally from a Jedi that was killed? Wouldn't it be the same color then? There's a pretty cool explanation!

According to Screen Rant:

Star Wars has always made the affiliations of its Force users very clear: the dark side has red sabers and the light side has all the others. This is useful from a visual perspective, it helps the audience know who’s who, but is there a functional reason? Why doesn’t a Sith ever choose to use a blue or green lightsaber? Ahsoka provides a very clear answer to this question: because they can’t.

Kyber crystals choose Force users and present themselves to their desired owner, but dark siders don’t experience this same connection to the crystals. The only way the Sith and other dark side Force users can collect kyber crystals is by stealing them or plundering them off the bodies of their fallen foes. Even if a dark sider were to acquire a kyber crystal from a green or blue, the act of bending the crystal to their will in a new lightsaber will cause the crystal to “bleed,” turning it red. "

[via Screen Rant]