Usually coloration in most plants intensifies with more UV light. That's generally why plants grown under grow lights have more coloring in the leaves and flowers. Similar to why UV causes people to tan. Maybe the partially shaded areas block some of the light spectrum which prevents intense coloration and allows UV to get in there and do its thing. I've gotten my worst sunburns on hazy, partially cloudy days. UV rays still get through even though other light rays get blocked.
Hope I didn't ramble too badly.