Over time, as the dinosaurs moved and their environment changed, natural selection lead to better and better generations as time goes on.  The first dinosaurs were most likely from Northern Africa. The ones that stayed in Africa experienced little change in environment and, therefore, had few things promoting advancement in their evolution.  Conversely, the dinosaurs that moved north and encountered different terrain, climates, wildlife, etc. were able to evolve and change into new and better species. Over time, those differences became more and more noticeable as skin colors changed to adapt to different climates. Some dinosaurs use  to feel cold more easily, hence the natural shift for those in the north to lighter skin.