I'm going to walk you through colouring fair skin, I will be using 2 Promarker, Soft peach and Ivory, the Image I'm using is by Mo Manning and called Birthday Party Paper Dolls.
Starting with the Soft Peach promarker colour where the shadows and darker areas would be on her skin, under her hair line, around the edges of her face, arms and legs, her cheek bones,her knees and elbows and under her chin etc.
You can now start blending with the Ivory Promarker, you can see on the face half coloured that you start by colouring over the Soft peach that is already on the image, working in small circular motion from the out edge into the centre of the face, you may need to go over the joining edge a few times until the two colours blend together, looking at the two images of the face you can see on the left image that it still looks a little patchy where the two blend, on the right image you can see that once it is dry it has smoothed out, this is because the ink still carry's on blending whilst drying, so on the second image you can see it is the right side of her face that is still drying.
Continue to blend the Soft Peach using the Ivory Promarker colour using a circular motion where the two colours meet until the whole image is coloured.
Lastly I went back over some of the shaded areas with the Soft peach shade as I needed where the deepest shadows would be to be a little darker.
I marked these areas with arrows in the image below so you can see exactly where I added the extra Soft peach colour.
If you compare both images the one above and the one below this text you will see the difference