Have a look for the Arquebusier (Journal of the Pike and Shot Society) Vol XXXV 2017. Stephen Ede-Borrett wrote a short article "The Colour of English Ordinance during the 17th and Early 18th Centuries".
In a nutshell "we don't know", but paintings from Europe show dark brown (bare wood?) and a colour akin to Austrian Napoleonic carriages. For Britain, there is the "fair ledd" reference (grey or red?). References for carriages being painted, but no colours. Firepower have a red painted carriage, but how much is original and how much is Victorian 'repair' isn't known.