What kind of paint to use for indoor windows to block sun?

If you just want to filter the sun, frosted contact paper is quick, easy, and durable. An added bonus is that it can be removed if your needs change. I assume that you could use more opaque contact paper for more light blockage.

there are films that have texture or other features to block out the sun 3 m and Gila are 2 companies that would be better than paint ,sold at lowes an home depot

I agree the frosted contact paper is far better. I don't even think there is a paint that you can use that wouldn't look tacky from both the inside and out.

you can tint school glue with food coloring and paint it on like stained glass.

We use a type of stick on film that we got at Lowes that blocks the UV rays. It was pretty easy to put on. I used Plaid Gallery Paint with a rose decal for another window for privacy. I used three coats of clear and 2 of frosted. That paint goes on by swirling your fingers in a circle pattern - 3 fingers at a time - all over the glass. Let it dry between coats. It seems similar to Elmer's glue but I've read that the glue doesn't work as well I think there is a difference between frosted contact paper that gives you privacy and the film that gives you UV protection from sun damage. HTH

I bought some window tint on a roll for my patio door from Menards. Works great. You can see out but you cant see in from the outside.