Skin tone

Here I am practicing coloring a face

It is important to establish where your light source is coming from as that will allow you to place the shadows on the correct area's

When painting skin you need to know whether you want more red, yellow or blue in the colour. As they are the main three colours that give us that flesh look 

Here I have slowly worked up overlaying colour to give me my desired outcome. 

Here is a general colour set in which you can use to create a skin colour

While searching for references I found this. When I said build up the colour this example shows one of the ways you can do this as it slowly gets more detailed from the line work to the final image

In relation to building on top of colour, you can use the same process for everything. Here is a more in depth tutorial I found in creating an realistic eye