Today I realized I no longer have any field grey paint, and began rummaging through my paint collection.

Does anyone made a good field grey out of tamiya paints? I seem to have just about every green, and grey they have made.

Also I do have Vallejo reflective green, and london grey which looks to be close...any thoughts?

Dont get too caught up in a search for the "perfect" field grey. Mix up some green and grey until you find something you like, and go with it. With all the different batches of colours used and weathering that the real thing went through ... your colour should work perfectly. If you have a scene with a few figures, it looks much better to have a few shades anyway. Works for me!!
Theres plenty reinactor sites as well as surplus shops on the net, to find some suitable references.

in shep paynes diorama book, he gives a recipe of colors to make field gray. if you have the colors its fairly easy. ive mixed oils to create a closeness to it.
its 3 parts black, 1 part blue, 1 part yellow and 6 parts white. i have followed this recipe and have gotten a pretty good color.