Reygadas wont reveal how it was done but he said it wasn't digital. It's 2 hours condensed into a few minutes and it wasn't a timelapse. So how was it done?
I think this film does the same thing. Though I still don't know how it's done lol. The trailer is here:
In the case of the short movie you linked to, I'm guessing a dark blue filter being gradually moved over the lens, on and off? Just like the old 1932 Dr Jekyll and Mr Hyde movie revealed the dark side of the character by passing coloured filters in front of the lens, with matching coloured make-up? (the same way gap-filling exercises in a school book can be done with answers printed in red, obscured by a red transparent sheet over it).
I think you're right. I think it's probably a specially constructed filter. There are filters that you can adjust intensity on it and it seems like Reygadas' cinematographer was using a very sophisticated one.