Little brick bungalow in the midwest US. I painted the interior maybe
25 years ago, still looks OK except for ...

Every 5 years or so, some roof sealant will crack or flashing will
separate, I notice wet spot on ceiling, proceed to fix roof and
Kilz the spot. After I was sure a couple spots were water-tight,
I got some standard-tint ceiling white paint from HD and covered
the Kilz spots. It was -much- lighter in color than the old paint,
which is faded and maybe has a little coat of nearly invisible
dirt.

I can take a sample of a painted surface to a Big Box and have 'em
mix a matching tint. I haven't done it in recent years, but it's
reasonable to expect it to match pretty well? This is question 1.

Question 2: How well might I expect it to work for ceiling white?
I'm tired of looking at the extra-light spots on the ceiling
where I applied the Kilz.

Question 3: If mix-to-match is practical in Q2, how can I get a
sample for them to analyze? Anyone had a successful mix-to-match
from, say, a digital photo?

Other ideas welcome.

TIA,
Will