Um, it might be because of the fabric - like I tried doing this on stretchy fabric and it came out brown. But yeah, leave it in for an hour and then wash it out. And if it's still too dark try doing it over again.
You're welcome! A friend of mine actually saw someone on YouTube with a bleached-painted hoodie, and when we tried to find tutorials and reference pictures, I was pretty disappointed at how little there was on the subject. So I thought I'd share what I learnt ^_^