There's a point where all teleportals get interconnected allowing you to warp to any teleportal no matter the color. I'm not sure if it's a specific teleportal or a number of teleportal found that unlocks it. For me, it was unlocked after I found a teleportal locked behind a storm missile block (in Burenia, I think?).
I am pretty sure the requirement is to enter Itorash or whatever the name of the very final area is. Maybe it's even required to use the save point there, but since that one is right at the beginning of the area, you might as well use it before returning to the main areas again.
I had a playthrough with all teleporters being used at least once and all Chozo Soldiers defeated, including the one right before the elevator leading to Itorash, but there was no unlock of all teleporters getting connected; hadn't entered Itorash though. Conversely, I got the unlock on another playthrough where I hadn't used all teleporters, but had visited Itorash already.