Hi there,
The only decent published picture of a grey O13 I can recall seeing is in 'Freight Wagons and Loads in Service on the Great Western...' by Russell.
The vehicle in question is W94004 which is in what might be described as a transition livery. It appears to be painted in a slightly darker shade than what became the standard shade. It also appears all ironwork including the underframe is grey. The lettering consists of : W94004 with 12T above it on a black patch to the left hand end (ie. standard early BR style) but with the tare on the right hand end in italics (GWR style). It is also marked 'Return to Fowey' on the middle plank immediately to the left of the drop door, again on a black patch. Finally, it has the diagonal ironwork painted white at the end with the door.
There is also a picture of W94071 with grey ironwork and unpainted planking, lettering is not all visible but appears similar to W94004 - this vehicle is (unusually?) fitted with a sheet rail. Most were not, but all would have run tarpaulined when loaded.
It may be worth pointing out that by 1948, all O13 appear to have had their original DC2 brakegear (as included with the kit) replaced with conventional independent brakes.
Hope this helps,
Mike