I have lived in Colorado for 55 years and worked in every part of the state in all seasons. The best fall color I have seen in all that time was Sept. 30-Oct. 2, 2016 along CO 17 on Cumbres and La Manga passes (just north of Chama, NM).
As others have said, the fall peak color varies from year to year, but generally starts in early Sept. in RMNP and continues through early October in southernmost CO and northern NM. So, the loop suggested by others is a great way to find perfect aspen color somewhere along your route.
My favorite early Sept. areas--Peak to Peak Highway (CO 119/72) and Rabbit Ears Pass (US 40). Mid-month--Kebler Pass and Marble area (CO 133) and Marshall Pass from Poncha Springs to Sargents (dirt road). Late month--Ridgway to Telluride (CO 62/145) and Lake City to Creede (CO 149). Earliest October--the previously mentioned Cumbres Pass-Chama area (CO 17) and the Spanish Peaks-Sangre de Cristo area (CO 12).
Enjoy Colorado!