Didn't know you hung out here
The book mentioned by Mike Cooper, 'Les Camions de la Victoire, 1914-1918', is a treasure trove of French softskins but with all text in French. I picked up a copy last November at the UK Nats in Telford. Let me know when we'll cross paths and I'll bring it along--AMPS in April maybe?
IMHO, if the color on the French 75mm and caisson in the Imperial War Museum is accurate, it definitely leans towards the blue side of blue-grey. I could send you a couple of photos if you like... If I knew how to attach JPEGs to this message, I'd do that for everyone's benefit.