So we know that the 10km ones are gold, the 5km ones are silver and the 2km ones have the orange stripe around the base. I’m not sure whether these differences are visible on the map before you pick them up however. Since I’ve been aware of the difference, I’ve only seen the 2km boxes on the map.
It would be useful to be able to distinguish because your inventory can quickly become filled up with 2km boxes, that don’t really justify the use of silver keys based on their aquisition rate. My preference would be to collect mostly the 5 and 10 km boxes for use with silver keys, and have a few 2km to burn thru with the gold key.
Here is how I manage my Portmanteaus: unless I have enough portkeys to use in my inventory to open them, I don’t pick one up in the overworld map. Also, to maximize points, I usually wait until my portkey inventory is full and fully unlocked before using them and collect items in AR mode and not before I use a Barruffio’s brain elixir. Hope this makes sense and helps 🙂