The color in the AL-31FN petals are a reaction between the heat from the engine and the alloys used to make the nozzle. Probably a reaction similar to how tempered steel changes color with differing temperatures.
https://en.wikipedia.org/wiki/File:Tempering_standards_used_in_blacksmithing.JPG
Heat and thrust are directly related. A higher thrust engine will produce more heat.
I also don't buy the argument that the nozzle will continue to change color over time. Can anyone show me Russian Air Force planes (using AL-31F) with color changing nozzles? Can anyone show me pictures of the Su-30MKK/MK2 nozzles changing color over time? I've never seen it.
Let me use pictures to illustrate the changing color of the J-10A/S nozzle over time.
Here are two pictures from the 2008/2009 time frame. These are dated photos from airliners.net. Standard blue/grey petals.
Now look at the multi-colored nozzles of the J-10A/S from Zhuhai 2016.
Here is a unique picture of an unmarked J-10A with very colorful nozzles. This one is very strange.
Moreover, please do not bring the J-10B/C into this discussion. We know AL-31FN Series 3 has higher thrust. I want to stick with the original J-10A/S using the 'old' AL-31FN.
Can anyone provide a credible explanation as to why the nozzle colors are changing over time?