Generally speaking, Star Wars Jedi: Fallen Order keeps a pretty tight lock on lightsaber colours. While you start off with a choice between traditional Jedi blue and green (or orange, if you splashed out on the bougie pre-order edition), there's never an option to go full Sith - and you can't unlock further colours until much later in the game.
As a big fan of Star Wars: The Old Republic - a game with a ridiculously varied collection of lightsaber crystals - I was curious to see whether someone would mod Fallen Order to accommodate these wacky combos. Sure enough, it's already been done: but the process is a little fiddly, and hasn't yet been streamlined into easily-downloadable mod form (of the sort you'd find on Nexus Mods).
Using third-party freeware Cheat Engine to edit the game, PC modders have written a script which allows users to tinker with the lightsaber's colour settings. By altering values in the code using the RGB colour model (and some hex), it's possible to create colours and combine them in different layers of the lightsaber for some funky designs - including black core lightsabers.