I wanna know how to kill specific modded items if the item has a color code name, like in this case it's called Katana Kuzanagi (the kuzangi part is red).
I tried doing §4 to make it red but that doesn't work.
The command I'm using is
/kill @e[type=item,name="Katana Kuzanagi"]
To select (and in your case subsequently kill) an item on the ground based on its name, you need to match the name string exactly.
This means you'd need to find out where the color sign is for it to be able to match. Ideally it should be after the space, but it could be before it, too, depending on how the author made it. You should be able to put it into an anvil to see the color codes position.
I recommend wrapping it in "", which is required for multiple words, and good practice otherwise.
In your case, you also misspelled the name of the item, missing the last "u".
@e[type=item,name="Katana §4Kuzanagui"]
/kill @e[type=item,name=(Item name)]