I put a little video together of a quick mixed media portrait I made in my 5x7" art journal. You can see it here:
I started off with a messy, mixed media background in my journal that looked like this:
It consists of layers of paint scrapes, oil pastel, collage and spray ink through stencils. Then I added a sketch of a face using a stabilo marks-all pencil and added "flesh tone" acrylic paint to the face and blending lightly with my finger. The trick is to use just a little bit of paint so that you can still see the layers underneath. Also, avoid coloring in the cheeks, eyes and mouth.
After letting the paint dry, go back with the stabilo pencil and re-draw in any details in the face that might have gotten painted over. You can also add color to the cheeks with acrylic paint (I've used neon pink), color to the lips with red paint pen and brighten the whites of the eyes with white paint pen (you can also use acrylic paint if you don't have paint pens). Add more scribbles and mark with oil pastels and a little bit more collage with tissue paper and other collage paper to blend the portrait back into the background a bit. You can re-draw in any areas again that were covered up by collage with the stabilo pencil.
Color in the irises with acrylic paint or paint pen (I've used Liquitex paint pen in aqua). I've colored the hair with the Mermaid Marker and then added a little bit to the irises as well (you can also use watercolor). Then I lifted the journal up and sprayed the wet areas with water and let the ink drip down a bit. Any areas that are too dark can be blotted with paper towel or collage paper. When dry, add a little bit of highlights with white paint pen to the bottom lip, nose, cheeks and eyelids. Also, when the eyes are dry, add a few little dots of white to the pupil for the sparkle in the eye.