Before you start

An SSN or ITIN
Form W-4 with your employer for federal withholding
Colorado Form DR 0004 for state withholding, if you want to adjust it
Records of foreign income and foreign financial accounts

Step-by-step

1
Complete the W-4 on day one
Colorado's withholding derives from your federal W-4 unless you file a DR 0004 to adjust it, which makes setup simple. Getting the federal form wrong is still the most common cause of an April surprise.
Via employerWho: YouFirst week of employment
2
Determine your US tax residency
The substantial presence test counts weighted days over three years to decide whether the US taxes your worldwide income or only US-source income. Your visa does not decide this.
OnlineWho: You
3
Check for an applicable tax treaty
The US has treaties with around 70 countries that can reduce or exempt tax on particular income. Colorado starts from federal taxable income, so a federal treaty exclusion generally flows through to the state return — unlike California, which ignores treaties.
OnlineWho: You
4
File federal and Colorado returns by 15 April
The tax year is the calendar year. Because Colorado begins from federal taxable income and applies one flat rate, the state return is among the simplest in the country.
OnlineWho: YouBy 15 April annually
5
Claim your TABOR refund
When state revenue exceeds the constitutional cap, Colorado returns the surplus to taxpayers — often as a refund claimed on the state return. You must file to receive it, which catches people who assume no tax owed means no reason to file.
OnlineWho: You
6
File an FBAR if foreign accounts exceed $10,000
Aggregate foreign account balances over $10,000 at any point in the year trigger a FinCEN filing, separate from your tax return.
OnlineWho: You

Documents you’ll need

Form W-2 from each employer, issued by 31 January
Form 1099s for freelance, interest and investment income
Passport and travel history for the substantial presence day count
Foreign account statements for FBAR reporting

Things most newcomers don’t know

Colorado's treaty treatment is friendlier than California's.

Colorado computes state tax from federal taxable income, so income excluded federally under a treaty generally does not reappear on the state return. California explicitly does not follow federal treaties and taxes that income anyway. For a researcher or student choosing between the two states, that is a real difference.

Source: Colorado Department of Revenue

You must file to receive a TABOR refund.

The Taxpayer's Bill of Rights caps state revenue growth and requires the surplus to be returned. In surplus years that refund is claimed through the state return. People who owe no Colorado tax sometimes skip filing entirely and forfeit money they were owed.

Source: Colorado Department of Revenue — TABOR

Denver's occupational privilege tax is small but easy to miss.

Denver charges a fixed monthly 'head tax' on employees earning above a threshold within the city, matched by the employer. It is a few dollars a month rather than a percentage, so it barely registers — but if you are self-employed in Denver you are responsible for both halves and for registering.

Source: City and County of Denver — occupational privilege tax

Your visa does not determine your tax residency.

The substantial presence test — a weighted day count across three years — decides whether the US taxes your worldwide income. Someone on a temporary visa can be a US tax resident; someone on a long visa can fail the test.

Source: IRS — substantial presence test

Common mistakes to avoid

Not filing a Colorado return in a year you owe nothing, and forfeiting a TABOR refund.
Assuming your visa type settles your tax residency.
Being self-employed in Denver and not registering for the occupational privilege tax.
Missing the FBAR because a home-country account did not feel 'foreign'.
Believing a filing extension also extends the payment deadline.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.