primer

I sprayed a few minis with black primer and interestingly I noticed that it's much easier to paint on top of sprayed paint than it is over brushed paint. Sprayed paint seems much rougher and new layers seem to grip onto it easier. I haven't used white spray primer; does it actually stick onto...

I was hoping some of the more experienced (or at-all experienced) painters out there could help explain when the different primer colours are used? Everywhere I look it is just stated to used the primer colour 'as required', and I can't decide what that means exactly. (I'm also colourblind...