Honestly I don't get it. What is the point of going to the trouble using a traditional stain then adding a synthetic? Why not just use the synthetic stain to begin with? The only problem I have had with Nitrate of Iron was when using a store bought stain that was adulterated with hydrochloric acid instead of using straight Nitric. This will brown and blacken the stock increasingly over time. The ones I have done with REAL Nitrate of Iron invariably turn reddish, though I have one stock that a large area on the cheek side of the buttstock that simply "didn't take" after drying overnight and before blushing and I simply reapplied and it was good. Nor does it matter what the source of iron is, modern steel, old wagon tire, finishing nails, no matter will test the same on the same test strip. Nitrate of Iron Crystals as well in testing by a Guild member. Also there is no need have a stain that is significantly acidic. It can be completely depleted and works fine. AND if it DOES seem to be a little too gold colored in my experience putting on a real boiled, driers added, acid neutralized LS oil finish will darken and redden the color over a week or two. But if plastic finishes are used this probably won't work. To me green is either some anomaly in the wood, poorly made iron stain or its something done in preparation or application/blushing.
Dan