In the case of The Rise of Skywalker, that yellow/golden color represents a blending of the green and red lightsabers. It’s a neutral color, one that depicts a “balance” to the force.
Where did Rey get the yellow lightsaber?
While the movie doesn’t explicitly say, it looks like she crafted the lightsaber herself. After burying Luke and Leia’s sabers, sending them into “Ghost Force” territory, Rey is reborn as a new kind of Jedi.