This is a topic discussed by many but all have the same answer. Black isnt Black, White isnt White in fact any color you use should not be its original raw state. Its all to do with scale reletivity. Ratios also apply to the scale you are building in proportion to distance you are pertraying. To describe the principle in depth would take ages and may paragraphs. A rule of thumb would be to tone down the Black, White, Red, Blue and Yellow to a contrast of the main color that you are using. I say these colors because they are the most common that are used for insigia and Battle stripes.. Red and Blue are added to Black, Red and Yellow are added to White. Just how much of these you add depends on the tone of your models main color. I would suggest that you have a browse through U Tube and see what you can find. It would be better to see this than someone try to explain it. But remember this, when mixing these colors always add very little drops to your Black etc as it doesnt take much to tone it down, if you add too much you will end up having to add more Black again and before you know it you have ended up with a lot more paint than you wanted in the first place.
Someone else may have a different take on this to me, but then, i do build on the larger scale that most on here.
Good luck
Andy