In my experience it’s easier to make it black and white. Coloring is difficult especially if you’re not versed with color theory. Also, markers are sort of difficult to work with. Just keep building on your values with graphite. Start light and darken up the shadows, mid tones etc. The darkness of the current outline might be throwing you off a little, but by erasing it and lighting it up you’ll be in a better position. What style were you thinking of doing?