I planning to start painting tamiya M-40 SPG gun crews.which color should I use vallejo US army field drab 873 or tamiya field drab?
My own judgement,is that vallejo US army field drab are far more suited for highlight or I am wrong?

In my opinion, you should use a mixture of colors since American combat uniforms faded and often had different dyes from different dye lots. Often, pants and shirts were very different colors depending on age and dye lot. A old fatigue shirt or pants could almost be a light grey-green and a new one could be a dark OD Green. Mix and match colors on all your figures. If your crew is set in Korea, they could be wearing the wool uniforms which were a olive green color.

Ditto !
I depend on the Lifecolor US Army uniform set.
J

"Field Drab" as used by the US Army was a brown shade, available as a paint, but not used for clothing.

The term Olive Drab was applied to nearly all articles of clothing and gear, but it is fairly useless, as it could be applied to a myriad of green and olive shades. The fatigue uniforms used after World War Two were a dull, slightly grayish green usually without any real olive (i.e. brownish) tone. It faded badly, and became progressively more chalky in color over time. Your best bet is to make sure that no two uniform items on your figures are ever exactly the same shade. Add a drop or two of black or white or any primary or secondary color to vary the base green.

If this all sounds maddeningly vague, try a Google image search for the term "Korean War US Army Uniforms," and you'll get a better idea of the range of shades seen in real life.

Richard,
Although I agree with what the others said, I sense that you were looking for specific paint recommendations. For US uniforms from later WW 2 through Vietnam, I like these two Vallejo shades as a good starting point:
Panzer Aces 318 - US Army Tank crew
Vallejo Grey-Green - unfortunately the ref number on the label can't be seen

These colors may appear too light when viewed in the bottle, but will provide a proper scale appearance once on a figure. As others have stated, you'll improve your results byyour results by using slightly different shades of these base colors on each garment. Good weathering will also help for units in the field.

Good luck,
Kurt