I've used peroxide on my hair quite a bit in conjunction with henna - my hair's still shortish and so I've been willing to experiment more. I'm pretty sure you've hit the nail on the head; that the peroxide is lightening the hair itself. Before my last trim, some of my ends looked translucent when held up to the light, yet they were definitely still henna-red - and besides, I've never had much, if any dye runoff after a peroxide session.
It's great to know that sun-in can offer that extra lift for people who have applied just one too many times (or fallen asleep with the henna on - it's easy to do!). I'm really pleased this has worked for you!
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