It really may be best for you to seek consultation with an attorney in your area.  Generally, an annulment is known as a declaration of invalidity of a marriage in Colorado and is a bit more difficult to obtain than a divorce since Colorado has what is known as a "no fault" divorce provision.  This avenue is typically more appealing to those that wish to avoid a divorce for religious reasons or to reinstate benefits lost upon remarriage.  The legal grounds for declaration of invalidity of a marriage are specific but one may be interpreted by the courts to lien in your favor for use.  You are correct that there are time limitations placed upon using these grounds in this way, some as soon as within 6 months of learning about the grounds.  And serving him with notice of you do not know where he is can be an issue.  The court must have "jurisdiction" over a party to be able to decide their fate. Seek legal help. Good luck.