If you're unable to file your Colorado income tax return by April 15, 2019, you may file under extension. This means you'll have extra time to file your return - it'll need to be filed by October 15, 2019. You don't have to submit an extension request to Colorado to file under extension.
Be aware however that the extension of time to file DOES NOT allow you to extend your payment due date. You must pay at least 90% of your tax liability by April 15 and the remainder by October 15 to avoid delinquent payment penalties.