Q. How I would go about paying withholding tax from an out of state retirement account, reported on a 1099-R to the Colorado Department of Revenue?
A. There is no way to do that. As Critter-3 said, you make "quarterly estimated tax payments", instead.
The only way to have "withholding" done is to ask the payor (retirement system) to do it. In your case, they've said no.
Another alternative is to increase the state withholding on any other income you have. For example, social security.
Note that you do not make quarterly estimated tax payments for each individual piece of income (e.g. your pension payment), but for any withholding shortfall on your total income.
There will be no underpayment penalty, on Colorado income tax, if the required annual amount to be paid (withholding + estimates) is the lesser of:
• 70% of actual current year net Colorado tax liability, or
• 100% of preceding year’s net Colorado tax liability
Reference: https://tax.colorado.gov/sites/tax/files/DR0204_2020.pdf
