Insufficient shielding gas or excessive heat input during welding causes oxidation of the weld and heat-affected zone, producing different colors. General interpretation: · Silver-white: good shielding, extremely thin oxide film, corrosion resistance essentially unaffected; · Pale gold/light blue: mild oxidation, usually acceptable, local pickling if necessary; · Deep blue/purple: heavier oxidation, reduced corrosion resistance, pickling and passivation recommended; · Gray-black/flaking: severely inadequate shielding, oxide layer must be removed and rework evaluated. Acceptance recommendations: Agree on the acceptable color range per project or standard (e.g., AWS D18.2 color chart), specify pickling and passivation requirements and inspection methods, and avoid subjective judgment by visual inspection alone.