I'm going to the game tonight, can I bring my hoodie (of course Rockies purple) and sun flower seeds, and a bottle of water and place the items in a backpack?

Yes the bag of sunflower seeds cant be open, hoodie yes, and water cant be open either. If plan on buying peanuts get them outside of the park little vendors have them for two dollars and are the same size. Back pack cant be huge aswell

As Long as seeds or water are not open as for the hoodie your choise

I think the dimensions of the bag needs to be smaller than 16x16x8. Check the Coors Field website.