I haven't done WP paint in a while. Unfortunately it requires an expedition to hardware stores, and doing a little test painting. The big problem is finding it in flat (lusterless)
The stuff I used to buy was Derusto brand 864-D-28 Seafoam green, but it was gloss.
I have a can of Military touch-up made by Illinois Bronze Paint Co., Lake Zurich IL 60047. It is called Styrenated Alkyd Lusterless Enamel, Green 34558 (595 A) meets Fed Spec TT-E-516A. It was probably made on contract to the military, so there typically won't be stock on it, and the environmental emissions laws might make it tough to get.
If you do score some, let me know, because I have a few 155 WPs to do myself.
Otherwise, Walmart, and other real hardware stores. The internet says Sherwin Williams bought Illinois Bronze and Krylon. if you do have touse gloss, you can give it an overcoat of satin clear to take off some of the shine, or if you airbrush, you can mix a flattener with the paint before you shoot it.
Another source might be the local Military vehicle collectors. There is a guy that comes to the conventions here, that sells military paint. He will have WWII OD, current colors, etc., so you can try that route. If you find a good source, let me know. Good hunting.
John