QUESTION:
A true color photograph requires three separate exposures with a red,
green, or blue filter. With millions of pixels involved, how difficult is
it to take those photographs so that they perfectly overlap?

ANSWER from Jeffrey Hayes
   You are right that a "true" colour photo requires 3 separate filter
images and recombined in the appropriate way.  It turns out that, with
either a cross-correlation technique on a computer (essentially self-identifying the
same features on the 3 images), the task goes pretty quickly -- a few
10s of minutes.  (This is for a typical CCD of say 500x500 pixels.)  The
harder part is really getting the colour balance to look "right" -- not all
computer monitors have the same colour response -- you really need a 24 bit
display to get it right.