I "officially" moved (got my driver's license and registered to vote) in January of 2017 even though my mailing address was changed in August 2016 (from Colorado to Washington). Should I file two state tax returns? If yes, would I file a nonresident return for Washington or for Colorado?
File a return as a resident for the state where you were physically domiciled (resided) while you worked.
If this state was Washington, then you file that state as a part year resident and report all income earned while living there in 2016, regardless of your "official changes".
If no money was earned while living in Colorado, then there is nothing to file or report. If money was earned in the first part of the year before the move then you file this return as a part year resident to report only the income earned here.
File a return as a resident for the state where you were physically domiciled (resided) while you worked.
If this state was Washington, then you file that state as a part year resident and report all income earned while living there in 2016, regardless of your "official changes".
If no money was earned while living in Colorado, then there is nothing to file or report. If money was earned in the first part of the year before the move then you file this return as a part year resident to report only the income earned here.