How Colorado withholds on a bonus
Colorado publishes no separate supplemental rate because it does not need one: the state's income tax is a flat 4.4% of federal taxable income, so a bonus paid separately is withheld at 4.4% and a bonus added to a regular paycheck is also withheld at 4.4%. On a $10,000 bonus the percentage method withholds $2,200 federal, $620 Social Security, $145 Medicare and $440 Colorado, leaving $6,595. The only difference the aggregate method makes in Colorado is on the federal line, which rises to about $2,700 for a $60,000 earner paid every two weeks.
Two Colorado deductions that are not income tax also touch a bonus. FAMLI, the paid leave premium, takes 0.45% of wages from employees, about $45 on a $10,000 bonus. And the occupational privilege tax in Denver and four other cities is a flat monthly amount ($5.75 in Denver) that a bonus does not change. Neither is included here.
What the withholding means for your Colorado return
Colorado's 4.4% on a bonus is exactly the tax the state will charge on it, so the state line settles even. Any refund or balance on a bonus comes from the federal side: a $60,000 single filer's marginal federal rate is 12% up to $50,400 of taxable income and 22% above, against the 22% withheld, so roughly $750 of a $10,000 bonus's federal withholding returns. The Colorado paycheck calculator shows the marginal rates for any salary. Colorado's optional Form DR 0004 lets an employee adjust regular withholding, but a bonus withheld at the flat 4.4% is unaffected by it either way.