Can you change the default color of shapes in current Pages?
It used to be possible .... but is it now?
No. Even if you changed the color of a custom shape (or added SVG artwork) and saved it to My Shapes (as I just did), it will still be inserted as a b/w shape in Pages v14.3.
One can style a shape and then click the + symbol in the selected Shape's Style panel. I just added a blue gradient shape style in that manner and that allows a one-button click to apply that new style to an inserted shape.