I hadn't noticed this until you mentioned it George, so I've just had a look at a shed load of photos.
At first, I thought perhaps it was a different type of metal, or maybe heat-resistant paint, but further study has shown some aircraft definitely have a different tone in this area, whilst some don't show anything at all, and these were fairly clear photos.
A close up of a currently airworthy example doesn't show any evidence of different metals and, of course, lacks any staining, as the 'blower' possibly isn't used.
However, on those photos where there is an obvious tonal difference, they all follow the same, or a very similar 'pattern', presumably the effect of the hot gasses on the shape of the duct.
The B&W photo below shows the typical pattern, although not as pronounced, or as dark, as on some, and darker than seen on others, and I would suggest that it is heat discolouration, with maybe oil and dirt 'trails' also.
The colour photo is the preserved example at Duxford, and although not the best source of how it looked when in service, notice that the duct appears to be either the underside colour, or bare metal, without the 'Invasion' stripe covering this area, which might suggest, given the accuracy of the IWM exhibits, that the area would not be re-painted with the stripes, perhaps due to heat.
On mine, I'll paint it a greyish metal colour, with heat discolouration, leaving the area clear of the invasion stripes.