Welcome to the Serif Affinity forums.
That is not currently possible unless you have used global colors. In that case, if you edit the global color and change it all the objects with that color will change.
However, in 1.9 (currently available in beta) it will be possible, using the menu Select > Select Same > Fill Color. With that function, you can select one of the objects, and from it select all the other objects with the same color, and change them all.
