For the carriage wood; not blue grey, which is pictured often, rather a "darkish grey or "lead" colour.
All iron work was painted black (lamp-black) except "the metal ends of the shafts, the elevating mechanism, trunnion bearings, inside of the capsquares, axletree arms, linchpins and washers were not painted but 'kept bright' and lightly oiled."
Franklin provides the precise ingredients for the carriage and iron work paint, of which the primary ingredients are 112 lbs of ground oxide, of white zinc and 6 lbs of 'lamp-black'for the grey and for the black paint 56 lbs of "lamp-black'. Various chemicals and linseed oils were also added.
He also states that paint test were conducted to replicate the correct colour that resulted in similar colour to 'Dulux, Ebony Mist 3'.
A further sample of the original paint was sent to Humbrol Modelers Paints. The nearest equivalent for the modeler is Humbrol Matt Ocean Grey 106 (I use this colour for all my British guns and equipment), which is equivalent to revel 47.
Ref: Franklin, British Napoleonic Artillery, p.36-38.
If you can't or do not wish to use the above suggestions, a dark grey will do. I hope this answers your question.