I see this on a lot of machines. Green/lime DC04's seem particularly prone to it for some reason. Its almost like tobacco staining but it is deep in the plastic. It seems to be mostly on the exposed places so I concluded it must be sunlight that causes it - if a machine is stored by a sunny window long term for example.
I have never been successful in cleaning a machine that has this staining.
DC01's do it too. Later machines tend not to. Perhaps the pigments in the Malaysian plastic is different?