Colorado runs its own exchange, Connect for Health Colorado, instead of using the federal HealthCare.gov platform. Residents apply for coverage and subsidies directly through the state site, which also connects shoppers to Health First Colorado, the state's Medicaid program, when appropriate.
Colorado has expanded Medicaid under the ACA, so adults earning up to 138% of the federal poverty level generally qualify for Health First Colorado rather than needing to buy a marketplace plan. This closes the coverage gap that exists in states that haven't expanded.
Colorado has also experimented with a state public option style plan, sold through private insurers but with state-set standards, which can be worth comparing against traditional marketplace plans for cost and network breadth. Because it's a state-run exchange, enrollment dates and special enrollment rules can differ slightly from the federal calendar, so it pays to check Connect for Health Colorado directly.