I have a new cabin with interior trim made of basic, #2 yellow pine. 
This includes a pine wood tongue-in-groove ceiling, ceiling support
trusses, and window /door trim.  THe cabin is located in the
northwoods of Wisconsin.

As expected, the new pine in the cabin is yellowing with age.  (The
cabin is almost two years old.)  All of the pine was sealed with
several coats of a good quality, satin-finish, oil-based wood sealer
(not poly-urethane).  No color or stain was applied to the newly
installed pine prior to sealing...it is natural in color.

Questions:
1.  Why does pine on the interior of a home like my cabin yellow with
age?  Yellowing seems to occur even if the pine is not exposed to
sunlight.
2.  Does the yellowing continue at the same rate, or yellow faster
when new then slowing over time?
3.  For how long will the yellowing continue?
4.  Can the yellowing be stopped or hindered once the pine is sealed and completed?
5.  I've heard that if I had used a water-based sealer instead of an
oil-based sealer, the yellow would not have occurred or would have
been drastically reduced.  Is this true?
6.  After two years, I love the current color of my yellowed pine
interior.  Is there a way to stop the yellowing at this point so the
current color is maintained?  Probably not.

I ahve been looking around.  It may be the finish.  You might ask the
supplier, maybe even online.

This outfit has a friendly FAQs site and can be contacted.  They might
be able to tell you to what extent the wood itself darkens.

http://www.heartpine.com/faqs/

My brother has fir doing the same thing. It's been 3 years and it's
done darkening. It darkened fast in the beginning and slowed down as
time went on. We added a side bar cabinet in the exact same wood and
style of cabinet, the installer said it takes about 2.5 to 3 years to
match the existing cabinets.

Don't know if pine is the same as fir, but there you have it. And this:

Pine:
http://www.vermontmadefurniture.com/appreciation.html 
..."Is a very cost effective soft wood that has much character. Pine
inertly has knots, pitting, and darker streaking in it. Pine wood has
a natural tendency to mellow or darken with time. This darkening
process is not as extreme as that of cherry but old pine has a rich
amber hue that newly sawn pine will only develop with time. Pine is
very soft and is prone to scratching and denting. A scratch or dent in
pine will not color as will a scratch in cherry.

Temperature, Humidity and Sunlight:
All fine furniture, whether brand new or antique, should have as
stable an environment as possible. A fairly constant temperature and
humidity level should be maintained. That can be accomplished by using
a humidifier in the winter months and a dehumidifier in the
summertime. Moisture causes the wood to expand or contract. Even
though our furniture is built to accommodate such movement on rare
occasions a particular board may move in an undesirable way.

Intense, direct and prolonged exposure to sunlight may cause your
furniture to dry out and may even fade over time. Careful attention
should be paid to this, as there is no finish that can totally
suppress the sun's UV Rays..."