Whenever I paint a portrait, I find that it doesn't really matter too much which colours I use as long as there is a mixture of warm & cool and transparent & opaque though I do often tend to lean towards primary colours. In this case, I have worked on both of these portraits with the same palette - turquoise deep, napthol crimson, titanium white, Turner's yellow and burnt umber (Liquitex heavy body acrylics). I tend to avoid browns and blacks as I prefer to mix my own but on occasion I will incorporate a small amount.
What I find the most interesting is how many different tones and colours I can create with such a limited palette. Initially, when I began painting 20+ years ago, I only used ultramarine blue, cadmium red & yellow medium and titanium white. I still managed to create 1000 paintings (over many years) that were all very different from one another with such a limited range of colours. I do incorporate a broader palette now, particularly for the backgrounds of my paintings, and I think that layering helps to create a greater variety of tints & tones in each painting.