C.R.S. § 14-15-117

(1) The general assembly finds that some partners in a civil union may legally have their federal taxable income determined on either separate federal tax returns or on a joint federal tax return. Since Colorado income tax filings are tied to the federal income tax form by requiring taxpayers to pay a percentage of their federal taxable income as their state income taxes: (a) Partners in a civil union who have their federal taxable income determined on separate federal tax returns must have such income separately determined for purposes of the Colorado income tax; and (b) Partners in a civil union who have their federal taxable income determined on a joint federal tax return must have their state taxable income determined based on their joint federal taxable income.