Dear Paul,
What colours do you use for American vehicles and helicopters?
I have been using Vallejo US Dark Green 70.893 for Army Vehicles and Camouflage Olive Green 70.894 for the PBR boat (this used to be called Russian Green).
I think helicopters should be Brown Violet 70.887.
Its kinda complicated but I will explain;
I use Tamiya XF-74 which is the period 'Olive' green acrylic paint.
I then wash with one of those washes that when dry you can go back and add water to remove a lot but gives depth.
Then dry brush with the original colour.
Then dry brush with Tamiya Olive Green which is a lighter greener colour.
The dry brush with Valejo Iraqi sand a linen colour.
After that I may use an AK rain marks paint if I think its relevant.
Then I use a mud wash such as AK Dark brown mud wash.
I may use MIG powders although you have to use a fixative otherwise they disappear with handling or if you then varnish.
Hope that makes sense?
In reference to colours Marine vehicles, noticeably helicopters, seem to a lighter more jungle green earlier but seem to move onto the standard green mentioned at the start of this later in the war. However I have been doing the 1967 ONWARDS so have not researched anything yet.
I have some Trumpeter H-34 choppers to do and looking at the painting chart it says this.
Khaki Green Mr Hobby H80/54 - Acrysion N80 - Vallejo 924 - Tamiya XF51 - Humbrol 159
I will investigate the Tamiya paint and hopefully I have a few jars in the racks.
Hope that helps?