I know this subject is a bit controversial and I apologize in advance for any stomachs I may upset. But I'm a bit confused about how exactly this works.

For my own tattoos I've implemented antiquated African techniques as well as some new ones. Off and on for a year, I've been working on a crude butterfly a little larger in size than the lid to a 20 oz bottle of Vitamin Water. To aid the creation of scar tissue, I rub cigarette ashes into the slight incisions. To keep the wound from scabbing over, I use lotion (prolonged healing aids the formation of scarring). What results are lines that are bluish-gray in color.
What I don't understand is the process that creates the color. When healing is complete (about 1-2 weeks), my skin is completely smooth and unraised so no ashes remain.
I've always aimed toward raised scars, but either my incisions are not deep enough or I'm the wrong ethnicity (I've read that black people are more prone to such). And the coloring isn't consistently uniform. African tribes sometimes use wood ash in their tattoos but for the aiding of raised scar tissue not so much for coloring.
Why does coloring take place?

No one needs to nag about the dangers of doing this. I take all reasonable precautions and look at this the same as paying someone to give me a more socially acceptable tattoo. In fact, I see this as a safer alternative and I enjoy creating my body art. I just wish I knew more about why and how certain things work.
Does anyone know or have any guesses?

Thanks.