If you have an action replay put it in your DS and put your diamond version in your action replay. Go to the code menu and select enter new code. Name the code whatever you want. Then hit the next option to enter this code: 12068AC6 000046C0

Now your pokemon should be shiny when you start the game, but be warned. They will not remain shiny if you save your game and turn it off. You must have this code on if you want the pokemon to be shiny. Happy Gaming! :-)