In Colorado if the other driver was at fault, his or her Insurance company is responsible for all reasonable and necessary medical bills relating to the accident, but they are have no duty to pay those bills until a final settlement or judgment can be obtained.
In order to get your medical bills paid as they are incurred, the primary source for paying medical bills is with your own automobile insurance company, provided you have medical payments coverage. If you have this coverage then your automobile insurance company is responsible for paying your medical bills up to that amount. If you do not have medical payments coverage then you should look to alternative sources, the first of which would be any health insurance coverage that you have. Other sources may be such things as Medicare if you are covered by Medicare or Medicaid if Medicaid eligible in Colorado.
If the other driver was at fault, and your medical bills are paid by your health insurance company, Medicare, Medicaid, then those entities will have some right to reimbursement from the insurance company of the other driver at the time a settlement or judgment is obtained.