And that is the basic reply given by Tracy White in a 2018 thread on another board:

https://www.tapatalk.com/groups/hyperscale/1-48-tamiya-moto-tug-color-t498364.html

(He's near the bottom of this rather short discussion.)

In looking for photos, I have read a few other threads on some other forums as well. Several are automotive-enthusiast sites. Don't know the people there, nor how accurate their answers then are, so have just "taken everything under consideration."

One fellow stated as fact what my own photo searches seems to be showing...until sometime in 1945, deck vehicles were painted a blue color. Then, they switched to the light gray late war. Black and white Mototug photos show a very few of them in a dark color that could be blue. The majority are in a light color that looks to be the light gray. And it took a while for Mototug production to ramp up, so the photos where they are predominant on deck all tend to be very late in the war. Any other tractor type I see in color other than a Mototug is invariably blue.

Another "theory" discussed on automotive sites is that the vehicles were delivered from the factory in one color, then repainted in use. Per this idea, Navy vehicles were delivered in the light gray seen on cars, trucks, etc. on land. Once aboard ship, they were repainted--either to better match the flight deck, per Bill's idea--or the island/vertical surfaces per your thinking. Under this theory, they just stopped bothering to repaint them late war, so all those light gray Mototugs I am seeing are simply in factory finish without repainting.

A corollary to this is jeeps. Rather than bog factories down with multiple competing orders, one service was often selected to order "for everyone." In this case, jeeps were ordered by the Army, and it is apparently documented that the Navy got most--if not all--of them through the Army. They were then delivered in "Army green." Then, repainted once aboard ship. In automotive forums, it is common to state that a dark colored black and white image of a jeep is actually dark Army green, not a dark Navy blue. I found one black and white image of a jeep towing a torpedo bomber, and an obviously colorized image of the same photo with the jeep "Army green." A couple other--either badly colorized images--or poor quality color images, have shown Army green colored jeeps towing planes on carrier decks. Under this idea, they were delivered in green, then repainted once aboard, but--like the Mototugs--by late war, they did not bother to repaint them. So, they were left in green.

And related to this is a discussion we had here not too far back on the color of Navy helmets. It was established that the helmets were primarily (exclusively?) made by one manufacturer, under an Army contract, and delivered to the Navy in green. Then, regulations allowed the skipper to order them repainted any color he wanted. There are thus documented color images of sailors wearing OD helmets on a ship where the skipper did not bother with a repaint.

For whatever this is all worth.