To do this glitch, do the following:

1. Cast Return to Portal (with a portal set elsewhere)
2. Get crushed (in my case, I fell while out-of-bounds in the Upper Monastery, but other places should work too)

If the timing is right, when you reload, everything will appear white in color. This lasts until you exit out of the game.

Incidentally, one time I instead got the game over screen twice instead of just once.