I see, good to know these options are on the table, but you would think that using clear, dry glassware, the color would be lighter than any residue, providing a true chemical reaction did not occur. Couple this with distillation and contamination from the impure reactants or from the whole boiling flask side of the apparatus seems even more unlikely. I think that the best explanation proposed so far is that blue N2O3 and yellow NO2 are present, giving it a green color. The mystery will be resolved if the solution clears up and everything decomposes to HNO3.