It is rare and random. There is a way to obtain a shiny Dialga. Before you begin, save, then battle Dialga. If isn't shiny, turn it off. Repeat the steps until a shiny Dialga appears. This could take days, weeks, months, or even years. Just because it's shiny, doesn't mean it is more powerful than regular colored Pokemon. THIS WORKS FOR ALL LEGENDARIES!!!!!!!!!!!!! HAPPY CATCHING AND GOOD LUCK!!!!! ^_^ :)