US Army diesels in WW2?

I have a On30 Whitcomb diesel model that I’m looking to paint and decal for the US Army during WW2.
I doubt anyone here would know this but I thought I’d give it a try. I have seen very few photos of diesels in Army hands that weren’t standard gauge, and among them, most in WW2 were solid black and a few standard gauge Whitcombs were OD green from the photos I have seen (even in b/w it’s sort of clear).
A few, however, were painted a color that isn’t black or OD green, and I can’t figure it out from b/w photos.
I think some that went to Italy might have been light yellow as the b/w photos show a lot of brightness in some of those shots.
But the small ‘critter’ ones I have seen photos are either a darker shade of yellow or grey, as they have black writing that is obvious. I know that grey was a popular color for RR equipment in WW1 but I wonder if anyone has good SOLID info either way.
I don’t need conjecture; I could do that on my own. And color photos of diesels in WW2 are almost non-existent.
I want to be specific on the timeframe, I’m looking for the WW2 era ONLY. The Army changed paint jobs in the 50s, introduced yellow running boards and often branch insignia on the sides by then. They became more changed by the 60s and 70s.
I was at the US Army Transportation Museum at Fort Eustis last week and I asked the people there. Nobody had a clue.

-Lee

Flickr photo set of my On30 layout