I played up until Beastiary and played very little of that, but I have never seen this distinction, and the stats appear to be the same on poe.ninja
The green color is not new as it was introduced during the legacy league. That is a relic version of a unique. Currently in Delve league, you can get these by finding timeworn reliquary keys. Those open up the timeworn reliquary (at the map device) which gives up to 3 of any previous league specific unique items with this special green "foil" coloring.
In the Legacy league you could find ancient reliquary keys which granted access to the ancient reliquary. In addition to the green "foil" coloring these versions had legacy stats meaning you could find legacy variants of any unique item at the time (for better or worse).