Rob,
I'm not exactly sure what you mean as to re-entering the States and employing "blue routes" down through Montana and Idaho headed towards Colorado, but your mention of routes 2 and 200 tell me you're talking about entering at Roosville, BC on US 93 and picking up US 2 just inside Montana, then following it east (it's an east-west route but does trend southeast for some segments as it goes around whole mountain ranges), to MT Route 200, and generally south/southeast from there. I do understand what "blue routes" are--various more lightly used State and US highways--or perhaps solely State highways. If all of that is correct, a fine way to work your way southeast to CO from BC may include:
First, I'd skip the crossing of the northern Rockies via US 2 and would instead go down to Whitefish on US 93 to MT 82 and 83, to Swan Lake and Seeley Lake and pick up MT 200 at the southern end of MT 83. MT 200 to the west runs a distance along the Blackfoot River, which is Norman McLean's "A River Runs Through It" stream.
MT 200 merges onto I-90 just outside of Missoula. If so inclined, Missoula is a fine place for an overnight at a riverside motel with lots of good food, brewpubs, and rocking college bars within walking distance.
Some 20 miles east of Missoula is Rock Creek Canyon, traversed by Rock Creek Rd (FS 102). The first 10 miles upstream (south) is paved and developed, then the next 30 miles is directly along a fine, fine Blue Ribbon trout stream. There are 5 or 6 NF campgrounds along the route, with The Dalles, Harry's Flat, and Bitterroot Flat being among the nicest. A midweek pass-through will likely find the campgrounds and the water but lightly used.
At the far south end, Rock Creek Road crosses MT 348, and 12 miles of pavement to the east is the delightful village of Philipsburg, where some fine craft brew, ice cream, fudge, a good fly shop, and other goodies await you. An overnight at the (fairly Spartan) RV spots at the Philipsburg Inn motel puts you 4 blocks from all of the above.
Leaving Philipsburg headed south, take MT 1 past Georgetown Lake, through Anaconda, to MT 569 just outside of that town. MT569 is a little rough but paved, over the Continental Divide, to MT 43 outside of Wise River. East on 43 to Wise River.
At Wise River, south along the Pioneer Mountains Scenic Parkway, 37 miles, paved, with many scenic pull-outs, NF campgrounds, and disbursed sites right along the Wise River. The Wise River is home to voracious brook trout and arctic grayling, too.
At the south end of the Parkway, go west on MT 278 for about 10 miles just to get over a divide to reach Jackson, MT and have a good look at the Big Hole Valley.
From Jackson you may backtrack on MT 278 to Bannack or work your way due south along gravel roads, past Reservoir NF campground, to MT 324 just west of Grant, MT. The Bannack route brings you to the very well preserved ghost town of Bannack, the first capital of Montana Territory. Bannack is a gem for history buffs and one can easily spend a day there.
Just east of Grant along MT 324 a gravel road to the south is the northern end of the Big Sheep Creek Backcountry Byway, a 55 mile graded gravel road (with a few miles of somewhat rougher 2-track in its center, near Island Butte). Morrison Lake lies a couple of miles west up a spur route and is a beautiful stopover/disbursed camping site and a hot fishing hole.
Jump on I-15 at the end of the Big Sheep Creek Byway, stop in Lima for fuel and provisions, and proceed a few miles further south to Monida, and there take MT 509 to the east. MT 509 is a 60 mile stretch of graded gravel which is aka as South Shore Road. It follows the northern foot of the Centennial Mountains and traverses the entire Centennial Valley. The Red Rock Lakes NWR is near the head of the valley, just before Red Rock Pass brings you into Idaho a little west of West Yellowstone, MT.
For another night out of the truck, look at Elk Lake Lodge, a splendid off-the-grid layout of small cabins at the edge of Elk Lake, a super-fine supper and breakfast menu, and a fun little bar which closes at 9:00 pm because that's what time Jake (the proprietor along with his gourmet cook spouse Laurel) goes to bed at the lodge. The Lodge is at the head of the Centennial Valley about 5 miles north of MT 509.
Leaving there, I'd look to some pavement on down into Idaho, through Ashton, Victor, and Driggs, cross the Tetons at Teton Pass, skirt the worst of the craziness at Jackson, WY, and head down to Boulder, WY.
At Boulder, you can follow the southeast side of the Wind River Range all the way to South Pass City on graded gravel roads which follow the Oregon/Mormon trail.
At Atlantic City (near South Pass), be sure to take the Atlantic City-Hudson Road (BLM Road 2302), all the way to US 287 near Sweetwater Junction. There you'll be looking at some wide-open pavement southeast to Rawlins and I-80. At Rawlins, you can head east on I-80 or WY 230 south through Saratoga (hot springs!) which becomes CO 125 as you cross into CO. Staying on CO 125 all the way south to Granby puts you on US 34 for a drive over Trail Pass Road (Rocky Mountain National Park), for some way above timberline driving at over 12,000'. Either US 34 or 36 out of Estes Park returns you to the High Plains on the east side of the Rockies.
Looking forward to following your trip! Have a ball!
Foy