Hi Martine, firstly, well done for trying your first portrait! What is your medium?
You say that your portrait is of 2 African Girls; how have you mixed the colour for the skin tone?
This should offer a clue as to the colour of the shadow, but bear in mind that shadows can be influenced by what other light the shadow area may pick up.
If the shadow is across the jaw of one of the girls (I can only guess as to the shade and hue of the lighted skin colour), I would be inclined to use something like a weak Ultramarine, and use thin washes to build up the correct tone. perhaps try adding just a flick of a less bright red, such as Light Red, or even Burnt Sienna.
I'm a sucker for purple shadows, but in a portrait, more thought is needed.
Lets know what you think, and if you're still not happy, come back to me and I'll try to help further, perhaps if you could send an image to me.
Good luck, and very best wishes,
RSC Wigan / Warrington / St Helens