We are doing an all white kitchen remodel and almost all photos of white kitchens have a contrasting base island cabinet. Is there a design principal at play here? (also often contrasting island counter)

Question two: We are getting rid of our kitchen table and want to put in a long island to extend our counter space. The additional space will have 2 armless dining chairs pushed up to either side. Any proportional advice for deciding the island size or shape?