Henna On Permanent Dyed Hair

[Extremus]

I have permanent dyed jet black hair. I want to keep it jet black, although my natural hair is still black, but not as dark as jet black. My questions were:

Can you apply Henna to permanent dyed hair?

Which Henna is best?

Where can it be purchased?

I don't know anything about Henna, but I thought it must be good if it's raved about this much.

Please Help
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