Hello chazmash18
I do also have the white nets, but the option to change the color is available. And if I change to any other color(s) before equipping these stadiums, then the nets will be that color as well.
I believe that what I said earlier is correct, because there is no default for a specific stadium, and white nets are not an option, just a sign that no option has been selected. So instead of saying default, it just says none, and your net color setting applies across all stadiums.
So to clarify, the 2 stadiums you suggested, you are correct about their default colors being different than white, but Ultimate Team is not using stadium default, only the net color you have equipped, because every color option besides none, which is white, is available as a consumable and must be applied as such to become active in game.
I hope my explanation is clear enough and helpful, and yeah, you'll probably have to control net color yourself.
If you don't have the option to change the net color in those stadiums, then I can't help you, and you should report it because it would be an error.
All the best to you!