There is so much more to portraiture than just being able to draw a technically accurate face. But, one step at a time. Before you delve into the many possibilities of portraiture, you need to have a good understanding of value, light and form, how to construct a head and its features, and much more. Why not take my mini-course to learn the essential drawing concepts you'll need to draw anything, including portraits.
Everything that you have ever learned about drawing comes together to draw an accurate face and figure. They are timeless subjects that we are all interested in.
I can teach step-by-step drawing and first week we learn about light and forms, the second week we will do the coloring and have the chance to compare the drawing with the real source, if any to see the similarity and the difference.