Ok, I'm not very familiar with watercolours so take my advice with a pinch of salt. (I get on very well with acrylics). I'm curently working on a hub for a similar topic but if you can't wait...
The trick to mixing colours is to start with the major colour first and very slowly add more colours until you find the right hue. In this case, you want a very pale yellow (an ochre perhaps) with a hint of pink, yeah?
I have two recipes for "white" skin. Check out the hub "Quilters Keepsake - Artist for hire" ... I used these recipes to paint my sons, you can see a pic there.
RAW SIENNA + WHITE = a creamy skin tone a little darker than the colour of untreated pine (the wood). I would suggest you try this one first, it's easier to manipulate.
CADMIUM RED (2 parts) + YELLOW OCHRE (2 parts) + WHITE (3+ parts) = quite a red hue but adding lots of white with: a) touches of Naples yellow, b) touches of burnt umber, c) Cerulean blue makes some lovely tones.
I then use these master colours to mix several tones. Make highlights by adding more white or more water in your case. Make shadows from adding tiny, tiny amounts of a Cerulean blue, burnt umber and/or a cool red. Did I mention that you need a tiny amount of these colours?
Depending on your style, faces can be extremely challenging...there are many things that make or break a portrait. Don't be put off though, it may come naturally to you. Have a practice and play around with these recipes and be sure to tell us how you went.