Thanks both @MrToughGuy - that's what I always try, cheers. Even for irregular troops or civilians I tend to take 3 core colours and paint various parts of the whole group in those colours, then finish the remaining parts in whatever variety of colours I fancy. Seems to work pretty well. I'm terrible for using weird paints that catch my eye, so over time I've learned that it's better for me - in the long run - to sit painting away from my paint collection, forces me to think a bit more before I pull another paint out.
@VonLucky - I tell you what I love about 15mm sci fi, is that it's not dominated by GW or Infinity (and therefore their aesthetic)*. I actually don't think I could replicate this type of pulpy, not grimdark, slightly Traveller stuff in modern 28mm (i.e. not paying a fortune for Rogue Trader era minis) even if I wanted to. Might be wrong mind, I know that Blind Beggar, Microcosm, Axiom's excellent Colony 87 and Lead Adventure are doing good things.
edit:*I should add I'm not hating on GW here, just that the trickle down effect on the industry is that other brands tend to follow, so you end up with skulls, power marines, daemons or manga-skinny dudes.