The ICM early Model T ambulance kit describes the overall color as intermediate blue, while one finished model picture shows it as dark gray. Since French vehicles were painted medium blue during WW1, I’m guessing the blue color would be appropriate, especially since said ambulance was constructed there.
Compromise! Paint it blue/gray to account for fading and dirt/dust. There is this for what it’s worth: MODELIMEX Online Shop | 1/48 Ford T Ambulance M1917 | your favourite model shop
Ironically, I discovered that the color recommended in the kit instructions IS a blue-gray color! Well, that problem out of the way.