[Craftybutterfly (OP)]

If using henna hair dye would adding hydrogen peroxide to the dye mixture lighten the final color? Would it be better to lighten the hair first? Any suggestions would be appreciated.

[Bored chemist]

The easiest way to get a less intense henna colour would be to use less henna.

[Craftybutterfly (OP)]

I should have made my question more clear, the hair I am trying to color is brown and I am trying to get it to be a bright red. Using less henna would just presumably make it less effective. I’m trying to achieve a brighter red finished color.

[Bored chemist]

If you bleach it after you dye it, you will bleach out the henna.
So it makes sense to bleach it first.
(The usual precautions about hair dyes, patch testing etc apply.)

[adianadiadi]

Peroxide will oxidize henna and the molecules that are responsible for making hair red. That means henna no longer be effective.