There is so much to learn about colour.
Lets start with skin colour
I find that many contemporary artists use a very limited palette of colours when it comes to painting portraits. This is a personal choice. I prefer the more traditional/classical method... using a good selection of colours mixing them as I go along. I start with a base colour depending on the skin tone of my subject, mixing small amounts of varying shades for the facial landscape.
The warm colours are used to bring out the features by sculpturing in the colours and the cooler shades for receding and shadowing. Keep in mind that skin tones will vary, depending on the lighting in the room and/or the natural sunlight from the outdoors.
PRIMARY COLORS Red, Yellow and Blue
All other colors are derived from these 3 hues.
SECONDARY COLORS Green, Orange and Purple. These are the colors formed when mixed with two selective primary colors.
By mixing these colours in different percentages you'll get a wide selection of hues warm and cool. For more information check out the many books on the market titled Colour Theory.
There are artists that spend way too much time formulating the right colours. I prefer to mix colours as I feel with no rules or restrictions; just relying only on my observation and mood.
For the beginner I would first suggest a good understanding of the colourwheel; then go ahead and do as you please... this is all about being creative.
Keep connected... more to come on colour