Well- when I wrote that comment I’d not got to the end. Although I thought I did when I got to the room with black walls and celebratory spinning rays…
Prompted by what you said I went back and found the invisible way out, and made my way to the end (back to the start screen :D)
So I guess my answer is I didn’t- I think it’s maybe a bit too subtle (the side effect of a 1 bit palette) Perhaps something that would have helped me is some screenshots beyond that section to prompt me that I didn’t quite get far enough.