retina prints / sun prints do just that ...
it is what Nicéphore Niépce ( the guy who invented photography ) did ..
to get a "positive" take a film negative, make a photogram &c
put it in contact with fresh photo paper in sunlight, and leave it there for as long as it takes ...
contrary to popular belief photopaper doesn't turn black immediately, it slowly changes and prints ..
experiment with direct sunlight and open shade, cloudy and sunny days ...
you can do this with a camera too, butyou will have a negative ... put paper in whatever camera you have
( or make one with a box and magnifying glass, diopter or other lens ) leave the shutter open all the way with the paper
in the camera, after 45mins or a few hours ( i have done this with a camera open for 30 hours before )
you will have an image on the paper ... if you make a box -camera you will have to cut the back out of the box
and make a focusing screen ( waxed paper or tracing paper works great and costs almost nothing ) slide the frame in and out
of the inside of the box and tape it to the bottom of the inside of the box when your image is in focus ... cap your lens and
put your paper on the focus screen and make your exposure .. its a slow deliberate process
in the end ( both cases ) will have an images that are unstable, and not fixed.
if you fix it it turns white, if you develop it it turns black and if you leave it alone
it stays a print and eventually fades to grey ... i do these all the time ...
but they don't take hours or days to fade, they may start to change
in minutes and hours ... BUT ... you might be in luck ...
i have made some with liquid emulsion, 7 years ago and they still look perfect .. like i took them yesterday
i think your best bet might be to use liquid emulsion to do this, it will take longer to fade ..
regarding the fading ... even in a dark drawer under a stack of paper they turn grey, the liquid emulsions don't as much ..
good luck with your project !
john