Fading roof

My roof was installed 7 years ago. After 5 years I began to notice fading. 2 years later the manufacturer, Metal Sales Manufacturing Corporation denied my warranty claim stating something must have been applied after installation because the fading spreads from one panel to the next. So what could be causing this?

I understand what it looks like but, I can assure you that no one climbed on my roof to apply spray paint. The pitch is to steep for me to get up there and it would be ludicrous to think that vandals would go through the trouble. The installer adamantly denies putting anything on the roof and said he would also contact Metal Sales. Also, the number of patches appeared over time. They did not appear all at once. Then there's the question of why would someone use spray paint. There's no indication of scratches or abrasions that would warrant the use of spray paint. So it really points to the contractor who installed it? If he continues to deny it there's probably nothing I can do?

I did not apply touch up paint to the roof and the contractor who installed it adamantly denies applying anything to the roof. So is there any type of natural phenomenon that could have caused it? Otherwise, if what you're saying is true, the contractor applied something. The question would be 'why?' And if he continues to deny it, I suppose I wouldn't have any recourse?