Use a range of colours to give depth, aim for some purpley tones around the eyes and some grey-ish tones in the darkest shadows. Also, as =BDT466 said, starting with greyscale or B/W to get a feel for how shading falls in different lightings before getting into working with colour will be a huge help. As for shading techniques, I personally get some base colours down, for example the lightest parts and darkest parts on top of the main middle-tone and build them each up with lower opacity transition layers, and I often play with opacities and colours at the end as well until I'm satisfied.

gradients and transparencies

Blend Tool!

For realism, or just realistic shading, opacity truly is key. Using different colours when shading is paramount in achieving a stunning, realistic end result. Colours are what give a piece depth, shape and, most importantly, life.

However, when first getting to grips with the method, I'd suggest using only B&W, this is the easiest way to learn how it all works before you delve into colour theory and how certain colours work off of each other.

When learning, you need to lower the opacity to approximately 1-2%(this does vary but that's around the ballpark for smooth gradients) and keep building it up, layering from one end of the spectrum all the way to the other(Translucent-to-full strength). With experimentation you will begin to grasp how the space between each shape dictates the prominence of the shading.

eg. Black: the closer the outlining edge of each shape is to the others, the darker it will appear; thus creating a dark, abrupt shape/shadow. To simulate a smooth transition you extend the space between each object.

There is no one perfect way, experiment until you find a style that your both comfortable and pleased with. Just keep practicing and your sure to get the hang of it.

use different opacities, play with colors untill it works out for you

5 colors in lots of layers with opacities between 7% - 15%
3 colors in the same hue, but different saturations and brightnesses (HSB color mode ftw!) , 1 color with more red and less brightness and white on top
the first 2 colors are soft shades, giving the direction of the light, the darker skin tone and the white give contrast to the shades or highlights.

it's easier to explain with images

never done a vector portrait.