John
The first thing to say is that it is not finished yet, there is still a lot to do, mainly dry brushing now I doubt I will need the airbrush again.
To start you need 2 good photos one of each side, ther is loads on Flickr and Brian Daniels site.
These are the two I found for 31184 (three eleven eighty four)
The airbrush comes out first, mix up some black, metal cote gunmetal, a little blue and make it very thin far thinner than normal. Mix up too shades one very dark almost black so very little gunmetal and blue and one that will dry dark grey. Then try to copy the photographs with spraying it on the roof and sides and underframe. You won't be able to copy it exactly so don't try. Then after a couple of hours but no more than 24 hours use some cotton wool buds to clean off the paint you don't want, the windows will need a wipe for starters, on the body sides use an up and down motion to remove the paint. Once you have done that spray it again going over with the darker shade to get the blacker areas.
Looking at the photos you can see that some areas are almost black whilst others are just dirty with plenty of blue showing through, you can achieve most of this with gentle airbrushing, if you get too much on just wipe it off and start again a piece of kitchen tool dipped in thinners works very well.
For the front I got some dark brown mix, I think I used pheonix tack dirt or frame dirt either will do again very well thinned and just dribbled it down the outline of the connecting doors. Let it dry for a couple of hours then remove the excess with a cotton wool bud dipped in thinners (you will probably need 10) again always in an up and down motion and trying not to remove the brown in the grooves, you will if you press too hard or use too much thinners.
I also sprayed the bogie side frames and water tanks using the frame dirt and I did beofre I sprayed the black. Then sprayed black on the axle boxes and other oily bits.
Now it needs plenty of dry brushing with a light brown for the bogies and on one side of the roof a light grey as it is int he photo.
Richard