Hmm - first it's the basics I suppose, like yellow or blue etc, colours the pencils actually have. Then I make that colour darker, for example with orange, red, purple os something. And then.... use a completly different colour - like green on the skin or red on blue shadows. it's weird but it's logical - everything next to eachother will reflect the other's colour, it's just more visible when it's on a shiny surface like metal rather then normal fabric. This is how a drawing becomes more different and realistic than if you were to only use base colours and just darken those. You just play around with the colours and see what becomes of it!