Colorado multi-state compact tax regulation iv - applicability
Regulation IV 
For tax years beginning prior to January 1, 2009 taxpayers may elect the multi-state tax apportionment option set forth in Article III and Article IV of the Multistate Tax Compact and the regulations thereunder as those regulations existed prior to January 1, 2009.  For tax years beginning on or after January 1, 2009, taxpayers must file in accordance with sections 39-22-303.5 and 39-22-303.7, C.R.S. and any regulations thereunder.