You will need to file a CO non-resident return and allocate the income to CO for the workdays in CO. According to CO Code 201-2 extracted below, the wages earned while physically in CO are CO source income and taxable to CO. Further, CO filing instructions for non-residents
"A nonresident is required to file a Colorado income tax return if they:
- are required to file a federal income tax return, and
- had taxable Colorado-sourced income."
CCR 201-2
"(i) Wage Income.
Income earned as an employee for work performed in Colorado is Colorado-source income, unless a more specific rule below applies. “Performed in Colorado” means the employee is physically in Colorado when the employee performs the work.
(A) Telecommuting. A Nonresident employee who telecommutes from a location outside of Colorado is not working in Colorado and the employee’s income from such work is not Colorado-source income.
(B) Work Days. An employee’s income is apportioned to Colorado based on the number of days the employee works (“Work Day”) in Colorado.
A Work Day in Colorado means a day in which the majority of the employee’s work time for that day is performed in Colorado. Travel time to Colorado is included in calculating the Colorado Work Day hours, but travel time departing from Colorado is not included calculating the Colorado Work Day hours.
The denominator of this ratio is the total number of Work Days the employee works in the year. A day is not a Work Day if the work done on such day is de minimis. See example (II) below.
(I) Example. Nonresident flies from California to Colorado on Tuesday but does not perform any other Business-related work in either California or Colorado on Tuesday. Nonresident attends a 2 hour Business meeting on Wednesday, returns to California Wednesday afternoon, and works 1 hour in the California office.
Travel time to Colorado on Tuesday is considered a Work Day in Colorado because no other work was performed on Tuesday. Wednesday is not a Colorado Work Day because the majority of the work hours are allocated to California (flight to California and office work in California).
(II) Example. Nonresident prolongs his work trip to Colorado through the weekend. While in Colorado on the weekend vacation, the Nonresident checks his or her email and responds to a few nonsubstantive emails.
Such a day is neither a Colorado Work Day nor a Work Day anywhere."
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