Did you measure it with a silver spoon?? (from a garage sale...or Goodwill?). Make sure you use plastic or stainless steel for measuring, stirring etc.
Somehow something else got mixed in...probably in storage....So...if you keep you salt in an old container you used to keep old lead in....
Faint Green: Antimony or Antimony mixed with lead or tin (bullets, sinkers etc)
Blue-Green: Phosphates (old soap, laundry detergent, lawn or garden fertilizer)
Yellow-Green: Barium, manganese(II), molybdenum.
Emerald: Copper compounds, other than halides. Thallium.
Bright Green: Boron
Most likely reason: onyonlyx

Andy