"SteveB" writes:
This can be for several reasons:

The substrate. Putting it on different things. Different brands of
drywall. Kilz or no Kilz? Primer or no primer? Which primer? How long
has it been there, and how much UV rays from the sun has lightened it? If
it is in a kitchen or bath area, how much oil or steam has changed the
color? Paint looks different after it has soaked into a wall for five years
than that which is a week old.

I have kept those little color things, and gone back later and gotten
EXACTLY the same mix, and painted it on, and it looks different than the
paint on there. Even clothes fade. Car paint jobs fade.

NEVER EVER EVER EVER LOOK AT PAINT UNTIL IT HAS DRIED A WEEK. It takes that
long to get even close to the color it's going to be.


Here is another reason. Sometimes the paint manufacturers change the
base composition.

I recently had the following experience. I went to my favorite local
paint store to get another gallon of the paint that the old-timer had
meticulously matched for me several years ago. At that time, he spent
about an hour with me using repeated rounds of adding slight amounts
of color, shaking the can, painting a swatch, drying it with a hair
dryer, waiting, then seeing how it matched and continuing until the
match was perfect. All for a gallon of paint.

Well, the other day when he made a new gallon using the old formula,
the match was way off -- even against a test swab made from the
original paint and kept in a dark area.

It turns out that Benjamin Moore changed the formulation of its bases
to reduce the VOC content and that messed up all the formulas.

Luckily, my guys is skilled and he was able to save the gallon by
using his magic to adjust the color by eye over several iterations.

This guy is worth his weight in gold and he doesn't seem to charge any
more than the other non big box paint stores.