Typically if an officer suspects that you are impaired by drugs, including marijuana, they will only give you the option of taking a blood test after you are arrested instead of offering a blood OR breath test.

Before you are officially arrested, the officer will also try to run you through roadside sobriety tests, which you can refuse. Many law enforcement agencies have what they call "Drug Recognition Experts" who will run you through roadside sobriety tests purportedly designed to detect impairment by drugs. These tests typically do nothing for you except help the police build probable cause to arrest you, and they are completely voluntary, so typically I tell people not to do them.

In Colorado, if they arrest you, they can require you to take a blood or breath test. If you refuse the blood or breath test after you are arrested, then they will mark you as a refusal, which carries worse DMV consequences, including immediate revocation of your license.

If you are pulled over and arrested for DUI, make sure you request a DMV hearing within 7 days of your arrest to contest your revocation, and make sure you ask for the officer to be present for the hearing.

The police use a blood test - under Colorado law - the result is in nanograms - 5 nanograms is the bottom number before you can be prosecuted for DUID Driving under the influence of drugs. Here is a link that will help . https://www.denverpost.com/2017/08/25/marijuana-impairment-testing/