Marijuana may be legal for recreational purposes in Colorado, but just like alcohol, it could affect a person's ability to operate a motor vehicle safely. Therefore, police in Colorado will be on the lookout for drugged drivers, just as they are for drunk drivers. This means they could arrest a person for driving under the influence not only of alcohol, but also of drugs.
In Colorado, if a motorist has five nanograms of active tetrahydrocannabinol in their bloodstream while driving, then they could be charged with DUI. This is true even if the motorist has consumed marijuana for medicinal purposes. If a motorist is charged with DUI and ultimately convicted, they could face fines, incarceration, and the loss their driver's license.
While police can perform a roadside breath test if they believe someone is driving under the influence of alcohol, there is no roadside instrument currently available to detect whether a motorist has too much THC in their system. Thus, drugged driving charges are based on an officer's observations of a motorist's impairment. Such observations might be subjective, however, as even the opinions of officers who are trained to be "drug recognition experts" could differ depending on the circumstances.
Of course, the best defense to a DUI based on marijuana use is to avoid consuming marijuana before driving altogether. However, if you feel you have been unfairly charged with DUI based on drug use, it may be possible to fight such charges. Doing so takes a careful analysis of how the law applies to the facts of your specific case. Legal professionals can perform such an analysis and provide advice on how to proceed.