Very basic skin color for beginners
I like to say that this is not a technique to use on all the portraits you will paint, the best technique to get the most wonderfull portraits is the grisaille, but meanwhile you can practice with this few colors to paint skin tones.
The Colors I use
This technique is very simple, the most important about this is how you achieve a natural skin color with so few colors, here the colors you are going to use, titanium white, burnt sienna, ultramarine blue, that is it, we do not need anything else for the moment.
First we need to understand that this is not the way most profesionals works but if you are a beginner and you want results asap I think it is the best way to get it.
How to do it
The color of the skin you want to get it would fully depend on the amount of white you add to the mix, if you want a very white skin it is obvious you need more white, little bit of sienna and almost nothing of ultramarine blue, but if you want a dark skin color then you need less white more sienna and more blue.
If you mix just white and sienna you will get a very crude skin color by adding blue you are going to gray the mix and the result would be a more natural color.
The Drawing is important
Before to start thinking how to paint skin tones first you need to get a very structured face drawing, because if the structure it is not ok, not matter how good you get the volume or the color or whatever you can do to the portrait it wont look ok, be aware that the first stage is the structure. After that you need to know a little bit about volume, because with volume you will get the tridimensionality of the head, eyes, nose an all the parts of the face.
Here is an image of a portrait I painted with this few colors, at the end you can apply some glazzes to get a more color variations.
In this first excercise I use white and sienna and almost nothing of blue
This second excercise I use less white the same amount of sienna i use in the first excersice and a little bit of blue
For the third excercise I use less white and more sienna and more blue as a result the skin tone it is going to be darker.
I hope this simple advice will help you to start painting portraits and paint skin tones easily.