How can I change the color from which to which the gradient should be changing in Inkscape?
Another approach is, when the gradient tool is selected and your blue line visible, to click on the starting point, the small circle, and select a color, and into the endpoint, the square, and select a second color.
When those marks are selected, they turn solid, to indicate selection.
A third way to do it: Select the area with gradient and then select the color dialog, the brush icon in the toolbar, "Filling and Contur". The current gradient should show up. Then hit the small pen symbol in the bottom part of the dialog "edit gradient". In the tool config bar, a combobox should appear "Stopps" with your colors. You can select one of them and change it in the color dialog. Then select the other one to do the same.