First of all lets get the facts straight. When you get a tattoo you have to understand your dermatology. you have seven ( 7) layers of skin. The outline goes through 3 to 3 1/2 layers. The shading depending on what type of tattoo you are getting varies but for the most part shading and color goes in about 3 layers. The outline is like a damn and keeps the colors from fading outside of there area which happens more than one thinks.
Also understand some people have thicker skin than others, the more you are out in the sun the tougher you skin will be , the younger you are you will tend to have more oil in your skin which all of these things have to do with how long you will hold the ink before you will have to have the tattoo touched up and how long the tattoo will take to do.

You are not going anywhere near the bone , blood vessels, or even the fatty tissue !

Scars:
Honestly you can tattoo any skin covered flesh on your body. Scars can be tattooed over a lot easier only if you are having the work done by someone who knows what they are doing. It is very easy to blow the lines out when tattooing over scar tissue (meaning going too deep with the outline). Skin over scars tend to being thinner even though you have the same number of layers.

People with dark skin , African American, Nigerian, or Hispanic ( in other words people who have dark pigmented skin , no offense to anyone) :

You have to understand if you have dark skin pigment , the darker the pigment in your skin the less the tattoo is going to show up . The darker pigment will over whelm the lighter meaning yellow, blue, green, white, gray, red ect.  It all depends on how dark the pigment of your natural skin is.

You can tattoo most any part of your body from Eyeliner to genital areas.

Inside the mouth tattoos tend to fade quickly simply because they don't dry out enough to hold the ink unless it is DRILLED ! into the skin deep enough to become permanent.