I have a tenant who signed a 1 year lease. Bought a house and Moved out after 6 months.

They don't automatically get their security deposit back and you do not automatically get to keep it.  The question is do you have damages for which you can retain the security deposit?  It is possible that you can retain some or all of the security deposit as unpaid rent during from their last rent until you relet the property.   But, no matter what you do with the security deposit you must comply with the Colorado Security Deposit Statute or you tenants may bring suit against you for three times the security deposit plus the cost of their suit.   Good Luck!