From Colorado's point of view, you are making money "out-of-state" and it is subject to CO tax.
Since you are based out of Seattle, WA, that is where you are considered to be earning your income.
When you complete the Personal Interview:
- Select Colorado as your state of residence on December 31, 2016;
- Answer "No" when asked if you "Lived in another state in 2016?" That question only applies if you changed your state of residence in 2016.
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Answer "Yes" when asked if you "Earned money in another state?", that being Washington State/
