Well, it is hard to make any general statements about Colorado because there are many different ecosystems. First thing to know is that at the eastern edge of the Rockies the elevation is about 5000 feet above sea level. Most passes in the Rockies are above 10,000 feet and the peaks of many mountains are above 14,000 feet. Snow melt provides much of the water for plants at higher elevations and snow is not unusual any time of the year above 10,000 feet. The old timers on the RGS would joke there were two season at Lizard Head Pass, the fourth of July and winter.
The Chart in the other thread is hard to read so here is what it says:
Lower Montane Zone ~6000' to ~8000' Growing season 137 Days
Mean Air Temperature 47 degrees
Mountain Mahogany
Rocky Mountain Juniper
Douglas Fir
Ponderosa Pine
Lower/Upper Mountane Transition Zone ~7800' to ~8000'
Mixture of the two Zones
Upper Mountane Zone ~8000' to ~9000' Growing season 100 Days
Mean Air Temperature 42 degrees
Ponderosa Pine
Limber Pine
Lodgepole Pine
Douglas Fir
Aspen
Upper Montaine/ SubAlpine Transition Zone ~9000' to ~9400'
Mixture of the two Zones
SubAlpine Zone ~9400 to ~11,000 Growing season 47 Days
Mean Air Temperature 34 degrees
Aspen
Lodgepole Pine
Limber Pine
Engelmann Spruce
Above 11,000' there are very few trees, mostly alpine meadows.
That's a good cross section of the mountain ecosystems. Of course, as you say there are Cottonwoods in the valleys and other species of Spruce and Aspens in different ecosystems. And the Colorado Plateau is a high desert so you have a totally different ecosystem there.
You did a pretty good job with your last route. I would say go to a photo site like
http://www.pbase.com and search for the towns you want to model to get a selection of photos of the area.
Here is a very good site (It can be hard to reach sometimes)
http://www.swcoloradowildflowers.com/introductory information.htm#VEGETATIVE ZONES
William