Hi,
Martyn Welch uses a combination of Humbrol Metalcote 'Gunmental' (which look like Balcklead), and 'natural wood' or 'leather' (both Humbrol) colour to modify it, to produce a dirty/slightly oily sheen, and/or general muck - think of fine coal dust to give you an idea. It looks very effective on the roof of railway vehicles.
The modification to that using a mid to light grey to significantly lighten the Humbrol gunmetal will produce a light grey with a half sheen, which is quite near. A final coat of Johnson's Klear will slight darken it so take that into account when mixing the paint to start with. The 'Klear' has a semi matt effect and will also reduce the metallic sheen at the same time.
All colour perception is different so what you like/think is correct is your choice. The lighter colours anyway soon dirtied up, Have a look at the following Durango & Silverton loco 481 at this link
http://michaeldmccaffery.com/RGS COUNTRY PHOTOS FOLDER/Number481.jpg < Link To http://michaeldmccaffery....20FOLDER/Number481.jpg
That is a large image by the way so lots of weathering can be seen
Though they use what is virtually a silver colour. but the effect will be close to a hard worked engine.
Finally if you can find the book 'The Art of Weathering' by Martyn Welch it is a very good read, Waterstones list it, and it is published by Wild Swan. No connection with hime or them by the way.