Mixing paint in brush

I never use the cap. Just do it slowly. And if you work without a cap, pop the bubbles because they will pop themselves when you are over the artwork and specs of paint will be all over.
Yup, that's how I do it. And if using a largish amount of paint (for an AB anyway), I will backflush now and again throughout just to keep everything mixed evenly. Some pigments are a bit heavier and can settle a little during a long session.
As mentioned above reducer first back flush carefully with out the lid is great advice.... I have a couple of multi-colored ceiling tiles in my painting area :( and I think the lid to one of my airbrushes is behind a filing cabinetlol
I guess I'm the odd ball, I stir with a stick- mostly because I stir it up and pull it out to check to see if that is the color I was wanting. To each his own I guess.
that is how i lazily mix paint all the time also how i graduate shades of colours too. i normally have the bottom just covered in thinners (i work in solvents ) then i add my colours then cover the end with sponge and let the air bubble back into the airbrush to mic the contents.. if im working in a colour i will add a drop of black to darken it mix this way then spray , darken again mix this way spray ( you get the picture) then i have graduated shadows and fades with out building up paint over and over to do so.