This is especially important in Clip Studio Paint as there is no easy way to swap gradient directions (you can do it, it’s just not easy. The toggle is buried a bit in the gradient palette). Unfortuantely, there’s no swapper arrow and clicking on the colors just opens the palette to choose a new color.
As you can see, by default, “X” is assigned to “Switch main color and sub color”. This doesn’t mean the sub color is brought into the foreground, it means the sub color is selected. Not quite what I wanted. After a little trial and error, I figured out that the option I want is “Switch between main color and sub color”. This brings the sub color into the foreground and moves the main color into the background.