Little bit lost, client wants lighter hair colour

[MissMuggle]

I'm not sure. Would it not just make a strength between 9 an 6?? I would just never do this.
The reason I'm thinkin it would make a middle. It's like if I wanted a 7/0 and mixed 6/0 and 8/0 to make a 7.
That's the only thing that confuses me.
If they want something with an extra kick in it ten go an buy some 18% lol

sounds like they're(college) mixing peroxide the same way as mixing tints like you said. I've never mixed peroxides. If 3% ain't strong enough....use 6%.