Mes Amis,
I think that I've got the answer.
I didn't realise that my copy of "Napoleonic Artillery" by Anthony Dawson, Paul Dawson and Stephen Summerfield states on page 229 that "Wagons had light blue body, light brown roof and black wheels" which is very helpful.
And, the icing on the cake is that there are, albeit black and white illustrations, photographs and paintings showing caissons with what appears to be light blue bodies!
So I think that's settled!
Cheers,
Paul