From I-70, west of Idaho Springs, take exit 238 for the Fall River Road/CR 275. This paved road ends in the vicinity of a mountain village called "Alice," mostly a collection of summer cabins, etc. This is the same road that leads up to the well-known St. Mary's lake and Glacier. The road follows the Fall River in a generally NW direction. After over six miles miles, the road makes a second, abrupt switchback to leave the Fall River drainage and climb up to St. Mary's Lake and Alice. Turn left at this switchback onto "Rainbow Road," aka: FR 174.1. This is a graded, gravel road. The road starts out on the north side of Fall River, then about half way up, drops down to cross over to the south side of it. Just before making that crossing is a smaller pullout where you can park if you want to join a segment of the Continental Divide trail that heads up to James Peak from here. Coordinates are: N 39° 48' 40.45" W 105° 40' 02.32". This is the trailhead for our third route suggestion for James Peak.
If continuing on to the reservoir, as you near the Fall River Reservoir, there will be a road heading off to the left for Chinns Lake. That will be FR 174.1D. Stay right to finish the drive on up to Fall River Res. The last mile or so will be rougher and hikers will likely be most comfortable in a higher clearance vehicle.
A car camp is possible in the reservoir vicinity. There are also primitive sites all along the gravel road to the reservoir, but there are homes/cabins and private property, so be sure you don't camp in someones driveway. The best primitive camping opportunity appears to be when FR 174.1 crosses from the north to the south side of Fall River but there are cabins in the vicinity. See coordinates below. For Front Range hikers, overnight camping will likely not be a concern unless you're looking for an early morning start and want to come up the evening before.