MaryLou -

Since you are a newbie, a little info might help.
Although such a trip would be pretty easy for those posting already -
If you have little to no touring experience, more caution might be advised.

A few things to remember - -
No matter how experienced you are - you get fewer miles in the fall.
That's simple - - the days are shorter. So don't expect summer mileage.

A coat may not be needed - but layers would be more useful. Rain jacket/windbreaker.
Light polypro, heavier polypro, ski gloves, fleece headband, 1 pair wool blend socks.
Some kind of long pants - convertible or hiking lightweight nylon. Lightweight thermax tights.

If you are so inclined - an Aug 15 departure is ideal for the Northern Tier out of Anacortes.
(You can also access via Arlington, but Anacortes is just so nice.)
The Northern Tier has some brutal climbs right off the bat - so its not for wusses.

But if you do the NT, you can ride to Glacier, then down to Yellowstone.
You should get to Glacier in two weeks, max - then to YNP in another week - US 89 east side.
That would give you two more weeks of genrally goodweather to get to Denver by Sept 21.

Three options are available in Colorado once you get to Walden.
Cache la Poudre Canyon to Fort Collins is an incredible downhill ride, but you have the Front Range cities.
Trail Ridge Road in Rocky Mtn NP is stunning, but iffy late in the season.
US 40 over Berthoud Pass is the most direct, but has a good deal of traffic.

Have a safe trip.