Shocking Blue Is a Dark Cobalt Blue Hair Dye. For Best Results, We Recommend Lightening Hair To A Very Light Cool-Toned Level 9 Blonde Or Lighter. All Semi-Permanent Blue Hair Colors Can Interact With Yellow Tones In Bleached Hair To Create A Green Tint So Make Sure To Do A Strand Test Prior To Applying All Over To Ensure You Get The Results You Want! If You'd Like To Avoid A Possible Green Undertone, You Can Add In A Purple Shade Like Purple Haze To Off-Set This Effect.