Originally Posted by Alfons

My guess would be that the gasket or sealant that's common to all of those components tends to leach out the color to a small degree and then the rain, washing, etc., would cause that color to make those streaks. You can see a similar issue with some types of metal siding on houses, but in many of these cases, the color coating on the siding oxidizes and this oxide washes off with rain to cause the house streaks.