the current way Fountain Fill tool work isn't bad but could be improved.
Fountain Fill tool actual draw gradient from current color to white or vice-versa, then we can replace the white color with needed color if that's the goal (in most case it is the goal)
But actually ther should be some more mode implemented on the fountain fill tool, we could have:
Standard Mode: Actually the way it works in current state/days of CorelDraw
Monochromatic mode: in this mode the tool would draw the gradient from the current color to it's darker of lighter standard nature, ie: from light blue > dark blue etc:
Complementary mode: This Would make the Tool allow crazy but unprecedented way to create gradient removing the pain of searching for the complementary color of the one already, this could be really appreaciated by artists who often go crazy in color in order to deliver extravagant and beautifull artworks
For the monochromatic mode, some example can be seen at work in affinity designer (by serif) people using that software are really satisfied of the way the tool work globally, also a LIVE PREVIEW MODE could added to the took and make it even mode revolutionary
Thanks and Success