Actually the Sierra Nevadas supply little to the Colorado River drainage. Look at the Kern River draining much of the southern high Sierras. It comes south from the mountains and then west. The Owens Valley to its east still all famously goes to LA.
The Colorado starts North of Rocky Mountain NP and collects most of western Rockies drainage before heading to Lake Powell.
Grand Lake, Colorado, adjacent to Rocky Mountain National Park and Arapaho National Forest is at the headwaters for the Colorado river.