The "old dirt road" mentioned in the route is 50-100 ft past a sign on the rainbow trail that points you back toward the South Colony area. At first I wasn't sure that was it, since it is very overgrown, and the only thing to ID it as a "road" is the car-width space cut thru the woods.
As for the bushwhack up the slope, it's pretty nice as bushwhacks come, just steep. On my way down on the road, I was trying to eyeball a more direct route to gain the ridge, but I noticed from the road that the woods are much messier as you progress deeper into the canyon. Lots of down trees, ground cover, cliff bands, boulders to contend with. I concluded that I would still enter via rainbow trail, but maybe stay as south as possible while still ascending on the ridge: 1) because tree line appears to be a couple hundred feet lower as you get deeper in the canyon. 2) Because where I came out was too far north and had to angle back SW about a half mile to be back on track.
A few other things:
There is ample parking right next to the rainbow TH junction, so keep driving until you see the giant brown FS sign on the RHS.
Also, your GPS will not work in the forest. Too dense, so don't rely on that. You must trust your gut and just go UP.