Well sounds like you witnessed quite a sight!
Since you mentioned the plane heading towards Seattle, I would assume it was making an approach to SEA-TAC Intl. (SEA).
Planes have MANY different types of lights on them, and depending on the Aircraft Configuration, Model, and Year, you have a variety of external
Landing, Navigation, and Anti-Collision Lights.
You most likely noticed the Green Starboard (Right) side Navigation Light after the plane made a banking maneuver (in other words turning). It might
have appeared Blue, but I assure you it was Green.
Most passenger airplanes have at least one Red Light, some have two. There is always a Red Navigation Light on the Port (Left) Wing Tip. Many times,
especially on larger aircraft, there is also a Red Anti-Collision Strobe/Beacon placed under the belly of the aircraft.
Some Passenger Airplanes, especially smaller Turbo-Props have White Strobes which are added in addition to the Red Anti-Collision Strobe/Beacon so as
to make the smaller craft as visible as a much larger plane.
The Larger the Airplane, the more Running Lights it will have. Most larger Jets have White-Halogen Running Lights on the underside of the wings, and
any number alongside the underbody. Some newer Jets (Mosly 737s, and 787s) Now have HID Running Lights, and HID Landing Lights as well. These are the
same as HIDS on higher end automobiles (they give off the colder looking white/bluish light).
When an Airplane is on approach they will always engage their bright "Headlights" which are called Landing Lights. These lights are also engaged on
Take-off, and only dis-engaged when the Aircraft is out of the immediate Airport Traffic area. The landing gear has lights as well, and these will
always be engaged as long as the gear is down.
If in fact the plane you noticed was on Approach, it would have brightened its lights. If on departure the lights would have gone from bright to dim.
There is quite a dramatic difference between the Standard Running lights and Fully Engaged Landing Lights. The Aircraft basically goes from a flying
Christmas Tree, to a floating Spotlight.
Let me know if you have any more questions