I have searched EVERYWHERE - and cannot find a site that gives tips or a walkthrough on how to PAINT AND WEATHER to get the look of realistic CONCRETE for Bunkers and whatnot. Anyone know how????

PollyScale 505210 "Old Concrete" makes a good base. Really looks like old concrete.
I would start with that .
Depending on what look you are after..... maybe several washes with a raw umber and work some green pigments around the base or anywhere it might have been damp enough to allow moss to grow. Streak the sides with a dark pigment to simulate water and rain stains.

Cheers,
Charles

I addition to Charles input, I would seriously consider doing an image search for new concrete, and then obtain images of older surfaces. I would try to emulate the fresh concrete look and then through weathering processes, give it the aged look. Just a thought
Brad

Hi Conctete weathers at different rates depending on the climate , here is the UK it soon takes on a dirty blackish colour in steachs running down its surface also it has slight green ting due to the dampness, so in reality any thing goes just as previous posters say do a bit of research , hope it helps cheers ian