The sun was right for this process today, I'm playing with a batch of 6" squares bought from www.blueprintsonfabric.com. The squares start off a yellowy-green color, above I've arranged some of my paper twists on a piece pinned to foam core.
Next I weighed down everything with a circle of non-UV glass, placed in the sunshine and waited 15 minutes.
The sun exposed areas turn darker.
After a cleaning rinse this is what appears.
I tried using some photos I've printed on vellum for these results...
10 minutes in the sun for the above square.
A more saturated photo on vellum and 15 mins in the sun for the above.