Verified answer

You could try dying it again. I had an issue a few times where I bleached my hair (naturally dark brown, almost black) of course it became a pale yellow and when I tried to dye it blue it came out green so I dyed it yet again and then it came out blue.

Go and buy some blue dye and add a little bit to the Splat dye then re dye your hair

You have to dye your hair twice to pick up the color correctly.