Colo. Rev. Stat. § 42-4-209
Whenever the load upon any vehicle extends to the rear four feet or more beyond the bed or body of such vehicle, there shall be displayed at the extreme rear end of the load, at the time specified in section 42-4-204, a red light or lantern plainly visible from a distance of at least five hundred feet to the sides and rear. The red light or lantern required under this section shall be in addition to the red rear light required upon every vehicle. At any other time, there shall be displayed at the extreme rear end of such load a red flag or cloth not less than twelve inches square and so hung that the entire area is visible to the driver of a vehicle approaching from the rear. Any person who violates any provision of this section commits a class A traffic infraction.
C.R.S. § 42-4-209
This section is similar to former § 42-4-208 as it existed prior to 1994, and the former § 42-4-209 was relocated to § 42-4-210.