that is the $100,000 question... the short answer is you will not get a match the base coat out of a can... at least any of the dozens or so shades of white and light gray I've tried. Eventually I broke down and bought an airbrush.
I've seen people make very nice paintjobs out of a camolfague gray, and other almost white gray colors. One thing that I tried and it was close was to paint a base of "antique white" which was a yellowy off-white color then follow it up with a satin semi-gloss pure white "dusting". I kept dusting until it looked right - it "almost" covered comepletely the antique white. With that as a base weathered with gray pastels and washes. But as for a one shot out of the can falcon base coat - never found one I was happy with...
Jedi Dade