Make a Dispatcher somewhere (take Game Mode as example) and call it “OnSphereColorChange” add a int (GroupID) and bool (or Color) as parameter. Inside your Sphere BP get your Game Mode (Cast to you GM) and type “Assign OnSphereColorChange” that will create a Binding everytime the Dispatcher is called you get notified. Make a Int Var and call it “MyGroupID” and Expose it so you can Edit it outside or assign a ID during Spawn. In the Bound event simply check if the ID that comes in is the same as “MyGroupID” if that is true do your Color switch otherwise ignore it.
Now all its left is to Simply Call “OnSphereColorChange” inside your Game Mode and pass it the group you want to change and the random bool/color and all of your Sphere BPs get notified.
Good Luck and have Fun.