I was wondering exactly which paint make and shade you used for the RSH sidetank? The shade when weathered is just right for one of the NCBs Cumberland area locos.
It's Humbrol 25 - Matt blue. Sprayed over a grey primer base courtesy of Halfords. I think the trick is not to spray the blue too thickly so the grey tones it down.
Add a dose of filth (mostly Track Colour) first sprayed then wiped away and then sprayed again and the colour isn't bad.