The Feldbluse (or Heeres Dienstanzug) was essentially a 'combat jacket' to be worn in the field. Development started in 1933 at which time it had a field grey collar, which was changed to dark green in 1935 before issue began the same year (despite it being 'Modelle 1936'). The various changes to it only began after the war started. 'Feldgrau' is actually the green-grey colour of the jacket, the pants were 'Steingrauen' (Stone Grey) until 1940, when they too began being replaced by 'Feldgrau' ones. So it was the colour of the pants that changed, not that of the jacket.
A search using Feldbluse Modell 1936 (or M36) should bring a veritable avalanche of hits.