Deformation test

I was wondering what material will deform less when exposed to the sun's heat.

The rings in the picture measure 155/165mm(6") diameter and 10mm high. They are hollow, the walls are just 0.8mm thick. Weight 14g.
They hung in the bare sun for about 6 hours. (temp today in the shade 22°C). All the time one side was exposed to the sun.
Two rings were printed in PLA, one black, one withe. A third was printed in XT(black) and the last one in nGen(bleu).
Extra weight on each ring 0.5L water.
After 6 hours three rings were deformed and got a bit narrower(left/right). They shrunk about 5 to 6 mm.
The fourth ring was clearly less deformed and shrunk only 2mm, surprisingly it was the white PLA that got less deformed. I expected the two PLA rings would suffer the most from the sun's heat.

Color makes a big difference in heat generated by light. White will always stay the coolest as it absorbs little to no energy from the light. Black will always be the warmest as it absorbs most too all energy from the light. If you are using sunlight for your heat source then all materials would need to be the same color for accurate readings. I am sure there is a lot more to consider then what I mentioned though, as this is just my basic understanding of color, light, and heat.