This is down to warmth in the hair. If you have used Silver Blonde and the hair now has a green hue, it means there would have previously been yellow-orange tones within the hair. Silver Blonde contains blue pigments, therefore if these pigments are added to an existing yellow base, a green toned result will emerge. This is a standard outcome when silver, grey and metallic shades are applied to any yellow or brassy base.
However, the issue can be easily remedied. Simply apply Colour Restore Iced Platinum on a 20-minute development to neutralise the underlying yellow tone. Once this yellow tone is neutralised, the pigments within Silver Blonde will correctly display.