It was even lower than the U.S. growth rate of 0.5%. Growth had been slowing in Colorado for the past decade largely due to shrinking birthrates and more deaths as the population grew older. At least some portion of the domestic out-migration from Colorado is made up of recently arrived international migrants.”But without the reliable increase in net migration, Colorado increased naturally. That was the primary reason for Colorado’s population growth, according to the demographer’s office. Colorado’s population growth has been slowing for several years.