So after the first attempt of using ColourB4, I tried it again another three times. I used it once more for the times recommended on the back of the packet and then over a weekend I used it twice more, but left it on for three hours a piece. My hair stank of eggs, but it was no lighter after that extreme attempt.
So I organised for a strand test to see if I could dye over the henna with professional chemical help. I ended up sitting with peroxide on my hair for 30 minutes. This is what it looked like after that.
It was muuuuch brighter in real life. So then I sat with a dark blonde dye on for another 30 minutes.
And this is how it turned out. No brown in sight. It's way lighter though and I am now a solid ginger lass instead of having 'red' hair. After speaking to the hairdresser who dyed it, she said that since the bleaching and redying didn't work, I'm going to have to ride the roots and just wait until it grows out.
Seeing as it's been 6 months since my last henna dye, it's going to be a looooong time before I have normal coloured hair again.