This is excellent.
Your coloured portraits are fan-bloody-tastic. Your use of colour is outstanding; it's almost as good as some of the stuff Matisse has done. The way you blend your blues and greens is just.. wow.
May I ask you a couple of questions?
1) How long have you been working at this standard like this?
2) Have you tried actual painting? Or maybe an oil pastel piece? I think you'd suit the later perfectly.
3) And the largest canvas size you've used?
But yeah. Anyway.
Your coloured stuff is amazing. However, if you're looking to go into realism, you may have to keep on drawing skeletal structures, and the drawing the muscle patterns one them. You can then move onto the placement of fat, and how it lays, which is very advanced for someone of only 18.