I replaced my damaged drivers side rear light on my avant with a used one in good condition. Fitted it and works no problems. Comparing it to my original passengers side light they look slightly different shades. I know they look slightly different due to there being a foglight in one side and a blank on the other but the used light appears to be a darker red and the white reverse section looks different too. They were no signs of water ingress in either unit when I changed all the bulbs over. It was from a car on similar mileage and age and I haven't heard of the colour in lights fading before. All the part number matched too!
Any ideas anyone?