from what I can gather, they made their painting in grayscale first, then turned off alpha and colored over it with the actual colors so everything stayed contained, blended things, then applied some filters and overlays. vague I know, but that’s also quite old, and I think there are better ways to achieve watercolor effects nowadays:
default krita now comes with some really good watercolor effect brushed on it’s own, and @Pesi has put out an even more amazing set of watercolor effect brushes here:
and there is this video to show some of the cool things you can do with it:
https://www.youtube.com/live/_UW8n0vShzU?si=zdrOIjud5jWfOG-l