adding a material to a sphere is easy
adding it to a design is a pain in fusion
even hidden objects are being selected (red) while i try to select the spheres (blue) only
https://drive.google.com/file/d/1rvnq3cc-pv_Zwlve8dkBxPHC8u-K5QP2/view
so better face / object selection is really needed and ideally you should make use of the Browser too to select objects there too