Zeana, depending on what colour tones you prefer, you could consider the three primary colours in both a warm and a cool tone, and then just choose maybe three (or so) colours for a painting. I try out combinations first and mix on the brush when painting.
I have never used black, preferring to mix several colours to achieve it - e.g. a warm black, cool black, purple toned black, etc! Depending on how much of each colour you mix on the brush you can get different black tones to suit your painting.
But it all depends on how you prefer to paint. Some like to premix the colours, others, like me, select a minimum of colours and mix as you go!!!