The weird part of how Olive drab colors on tanks (and really any military vehicle) work is there's tons of variables. Was the picture taken in direct sunlight or shadow? Was it a gloomy day? How much dust has accumulated on the vehicle? The M18 GMC was introduced in 1943 so your color would be closer to A/N 319. This would be slightly glossy but definitely not shiny. A/N-319 is not far off from the No.9 Olive Drab used on early war vehicles but don't forget, different batches made on different days and runs would have some pretty startling variations in hue. One run could be fairly green while the other looks so brown that on a cloudy day it would seem black, but that wasn't common. So believe it or not you can actually play it fairly loose with how close the color is, because even in the same unit you would end up with a plethora of different shades of OD green based on when the paint was made, who made it, and how well the guy with the pigments did during mixing.

Source(s) Used-A brief chat with our museum restoration managerand: https://olive-drab.com/od_mvg_odpaint.php