Note: This is a tough alpine route with serious exposure to potentially fatal falls. Can be completed in summer with some means of self-arresting on scree (ice-axe, etc). Not for the inexperienced.
3hr. Head up broad flats on the south side of the Rainbow until beech forest starts. Stick close to the creek, heading upriver through beech, river rocks and the base of scree fans on the TR. Remain on the TR as the valley swings north, sidling in, or above, beech until you reach the head basin.
2 routes appear possible.
1) climb the scree gut direct to the western side of the pass – though bluffs near the base mean you have to sidle into the bottom of the gut. I’ve not tried this, but from the top it looks possible but very steep. The last 50m will be the worst.
2) climb the last grassed rock face on the west to the base of bluffs. Sidle / climb north along the base of these bluffs, crossing one tricky ravine. The bluffs shrink back to the ridgeline, and following thoir base up and west leads to a large rocky basin 20m below the western ridgeline. It is possible, with extreme care, to sidle from this basin to the western side of Rainbow pass (20m to the NE of the basin). Ledges are narrow, but stable with good hand-holds, but the sidle is very exposed and a fall would be fatal.