Im making a movie with cactuses for my thesis and my director wants to kill them for movie purposes. Instead I want to save them by maybe painting them with a washable treatment that would make them look like dying. Is there anyway that I can paint cactuses without killing them? Or any type of paint that might not be harmful for them?

I hope these cactus are not out in the desert...they are protected. Have you thought of fake cactus? How about cactus that are already painted (and dying)? You could repaint them.

Yes, I didn't really think about it until we moved to AZ, but some of the tall cactus can be over 100 years old. And as Daisy mentioned, they are protected. I would go the fake route or do something through video editing if you want to change something in your movie.

Its some cacti that we have already bought

You don't want to use any kind of spray paint, I think. Most kinds of paint are probably pretty permanent. There probably is some kind of specialty product you could use that would wash off, but I wouldn't know where to start.

Maybe there's a way to sepia just the bodies of the plants in post, or maybe just find a way to broadcast some fine dust all over them, which you could hose off later.

Wet them and dust them, sounds good.
There is a
temporary hair color that is like colored hair spray and will wash off. There's white here, and browns at the beauty supply store https://www.walmart.com/ip/Goo...

If this director has any education in film, he should be able to manipulate the color after filming or get someone else to do it. But people are stubborn. I would use hair chalk maybe, but I'm a plant newb. I don't know how long you would want to leave it on or how you would keep it out of the soil.