Once you know how to draw your fashion figure properly and have lots of practice drawing it, it's time to make it look alive by rendering the skin tone. The main goal of skin coloring is to make figures appear 3D and we achieve that by adding shadows and highlights. There are two ways to do that and one is using shades of grey with your skin color tone or the other way involves using 3 different shades of skin color.
Choose Your Light Source
If you want to represent your fashion figures as 3D models, then you have to choose one light source. It can come from the left or from the right side and will hit the figure in a diagonal.
When you decide where you want your shadow to be coming from, then you have only one rule to follow - the lightest points of the body will be on the side where your light source is and the darkest shadows will be on the opposite side of the light source. So if you choose your light source to be on the right side than your highlights will be on that side and your darkest shadow will be on the left side of the body.
Start With a Base Color
Although, I have seen some people do the shadowing first and then blend everything with the lightest shade, it is more common to start with a base color. This color will be the lightest shade and will cover the entire body but mostly be on the side where the light source is. When I use Copic Markers (Amazon Affiliate Link), my base color is usually E00 - Cotton Pearl.
Add First Layer of Shadows
All parts of the body that are on the opposite side of the light source will have a shadow color. This is usually a two shades darker color than your base color and for this purpose, I usually use E11 - Barley Beige.
Add Drop Shadows
You will have these shadows when something is blocking the light and casting a shadow on the body, like a piece of clothing or another body part. The best example is when you have a big floppy hat, it will cast a shadow on your face almost half way through. For drop shadows you can use one or two shades darker color than your regular shadow color. I use E13 - Light Suntan for drop shadows.