Do you have to file a report if you're involved in a car accident in Colorado? The short answer is yes. Colorado state law requires all accidents be reported to the law enforcement agency in the area, even if it's not reported immediately.
Any kind of incident that causes damage or injuries to a vehicle or the people inside is called a traffic accident by law in Colorado. Incidents where no damage or injuries took place may fall outside the definition for the purpose of reporting. Interestingly, when you do decide to report your accident, you can do so online or in person with an officer.
In cases where you're involved in a car accident during poor and extreme weather conditions, the online service can actually be used to alert police and medical responders about the accident. Known as an "Accident Alert" program, people can only file a report as a way of communicating the need for help if the area is in an Accident Alert enforcement state at the time of the accident. Otherwise, it's best to call the police to the scene or to wait and fill out an accident report online for general reporting.
Online reporting makes it easier for those who have simple car accidents to report it to police, because they may not have damage to their vehicles or injuries to those involved. There are some instances where you need to call for assistance, though. For instance, if you suffer injuries, if the other driver appears to be drunk or intoxicated, if public property is damaged or if the accident was a hit-and-run crash, then you need to call the authorities for appropriate backup and assistance.
Source: Colorado State, "Colorado Online Accident Report," accessed May. 07, 2015