can i put it on top of my dark blacck hair?

Yes you can put henna on top of dark black hair. I have black hair and I use this once a month. Your hair will not turn red though. It will just strengthen your hair a whole lot. If your hair is light black you may see some amber shine in the light. Henna isn't a "hair dye" it is a natural plant and it bonds to the keritain in your hair staining it. So it's not actually stripping off your color so if your hair is blonde you will see the red. If your hair is dark you may not see it but it is still well worth it. My hair is past my butt because of how strong it makes your hair. Which before henna I was lucky to get past my shoulders because of split ends.

I have medium-brown hair. What will the color be? Not as red as it is on the package right?

I have medium to dark brown hair. The first time I left it in for almost 3 hours and it was quite a difference. It faded to a nice color though. Subsequent touch-ups I only leave in for 45 -60 minutes. The difference isn't as striking. It still has a reddish tint to it, especially in the right light. However, it is not the color on the package. If you are concerned, you may want to condition your hair and then not leave in as long. There are two packages per box. You can always apply the second time if you want it more red.

What color is your Hanna Natural 100% Pure Henna Powder, 100 Gram? I have never used Henna before.

I'm not sure if you're asking what color the henna is or what color it will die your hair. The powder is green... its derived from the leaf of a plant. But it will die your hair red. Pure henna is always red. If someone's selling any other color and calling it henna, its not pure henna. You can read everything you ever wanted to know about henna at hennaforhair.com

Can this be used for tattoos?

it's 100% pure so yes, tattoo henna and hair henna is the same powder as long as it's 100% pure, they are just prepared different. With hair henna application you just mix with water. On the skin you prepare it with strong coffee, tea tree oil and some other things I can't remember off the top of my head... look up a recipe, Amazon has quite a few books on it too.