Denise,
Just remember to get the right Portra, I believe I said NC.There is NR which is higher contrast and brighter. The first is better at skintones. Some tips:
A. Shoot and paint in natural day light.
B. Make sure if you are inside, (if at all possible) that green foliage is about 50' away. Outside use an IB filter it removes the greens.
C. Get the biggest color photo of the subject you can afford.
D. Place photo next to subject so you can grab the skin tones from the model. This helps because the photo is always in the same position. I find this technique especially useful on children.
An example of this is on my website
www.portraitartist.com/knettell. It is the painting of a girl in a ballet skirt. The girl had a wonderful exuberant personality, she would sit still only very fleetingly. Working basicly from life with her I think I got an expression of barely contained glee. That would have been impossible using only a photo.
My caveat about 50' distance from foliage is well known among professionals. I had to learn it painfully. One of my first commissions was from life. The lady was beautiful, the portrait elegantly composed,the skintones tree color.