Lets get this straight.
First of all, when you're trying to draw something realistic, forget about the black lines. No black lines.

The main thing after no black lines is the colouring is the so called shading.
Usually people start off from a black/white drawing as a scetch, after that they add colour to it.
I like doing it like that, for example.

After that I start shading the whole thing, with different tones of grey.
And as a finish - colouring.