You will find that nearly every colour that you see isn't a pure primary or secondary colour-it's a mixture of all 3.
If I was going to paint a perfect blue sky, I would use a light blue, but I would still mix a little yellow and red into it, and of course vary these quantities across the whole sky area.
A good technique which i am using at the moment is to paint with pure colours directly from the tube and mixing them together ON the canvas or other surface (this works best with oils!) To start with you will have very intense colours all clashing together, but in the mixing process they become neutralised to some extent ( the more you mix the less intense they become) but still create a lively surface which simulates a natural effect.
You can use white, black and various earth colours (browns,greys, ochres etc) to tweak the mix to vary the tone and the intensity.
This probably all sounds a bit messy, and it is! but it's a good way of preventing your colours looking dead on the canvas.
I believe that Rubens and other old masters used some variation on this technique.