An automatic six-month extension of time for filing the Colorado income tax return is allowed for all taxpayers. However, an extension of time to file is not an extension of time to pay the tax. If at least 90% of the net tax liability is not paid by April 15, a late payment penalty plus interest will be added to your tax due. If 90% or more of the net tax liability is paid by April 14 and the balance of the tax is paid when the return is filed by the last day of the extension period, only interest will be assessed.
Also, if after April 14, you find that your estimate of tax due was too low, you should pay the additional tax as soon as possible to avoid further accumulation of penalties and interest. Pay your estimated additional tax with another Form 158-I voucher.