The colour and brightness of the light from eyes and wings depends on fairy's emotions.
The blue one was captured recently and she is scared, the red ones are there for long and they are deeply depressed.
In that world the old man and the boy are not the evil demons, they are just normal old man and boy. From their birth they were told that having fairies as pets is okay and killing them for their wings is also okay.
Fairies' wings keep their brightness and colour for a long time after fairy's death, so there are a lot of companies specialising in making fairies feel certain emotions ( =colour) before killing them to create lamps. In that world fairies for people don't mean much more than laboratory rats.
By the way, the cost of the detached wings depends on their colour. Blue and red ones are the cheapest and the most common for obvious reasons.Killing fairies in the wild is almost impossible, so people have to catch them using the bait. For this reason bright, happy colours are extremely rare and on sale for a huge amount of money.The rarest one is deemed to be the solar yellow. Only one case is known: two robbers entered the house of the factory's owner and told the fairy that they had killed her master.