Due to the nature of the problem, I've decided to post this on chemistry.stackexchange and not electronics.stackexchange, since I believe that the problem is chemical in nature.
Long story short - we have an electronic product that is submerged in fuels (kerosene being one of them) and uses an RGB LED (click here for datasheet). Due to a sealing problem in the enclosure, fuel has managed to get in and cover the PCBs. What's interesting is the effect that has had on the PCB. The PCBs functionality has been completely unaffected, apart from the fact that the red LED in the RGB LED module has completely stopped working. We've replicated this ourselves manually by submerging 2 new PCBs in kerosene for a day and then taking them out and powering them up and seeing that the red LED stops working entirely. The green and blue LEDs continue to work just fine.
Examination of the failed boards shows that there are no other electrical faults. It is just the red LED that completely stops illuminating.
Looking at the last page on the datasheet, the LED material is listed as AlGaInP / GaAs. Is there any obvious reaction between kerosene and these materials that would explain why just the red LED stops working?
Update After leaving the PCBs to dry, the red LED starts working again! So the problem is not permanent.