Field grey is an unfortunate name that has led many modellers astray over the years. The colour is actually quite green, although there were countless variations during the course of the war.
Most uniform items started out being manufactured out of very good quality wool, but as the war progressed more and more synthetic fibres were introduced and the quality declined. This reduction in quality control, and the natural wear and tear is what led to the wide colour variations. To add even more confusion, it was common for officers to have their uniforms made by a tailor, using much higher quality material, and of course one tailor's bolt of cloth was usually different from every other tailor!
All references I have seen, including modern photos of original uniforms, and colour photos from WW2 (always a slightly dubious colour reference source) indicate quite green colours. Some do not even apear grey at all, so I would not be worried by this.
As a footnote, I tend to use Humbrol slate grey (again a green colour) and shade with various tones of green and grey.
A good reference if it can be found is German Soldiers of World War two by Jean De Lagarde, published by Histoire collections. Also Camouflage uniforms of the Waffen SS by Michael D beaver with J F Borsarello.