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. When mixed with Pastelizer, Shocking Blue becomes a gorgeous steely, blue-grey pastel shade!