Can alimony (maintenance) be modified after divorce in Colorado?

Possibly.  If the parties' separation agreement provides for contractual maintenance (formerly known as alimony), which is typical, they likely have agreed maintenance may not be modified later.  If there is no such agreement, for example if maintenance was resolved by the judge issuing permanent orders, CRS 14-10-122(1)(a) applies and states in relevant part:

maintenance may be modified only as to installments accruing subsequent to the motion for modification and only upon a showing of changed circumstances so substantial and continuing as to make the terms unfair, ...   The provisions as to property disposition may not be revoked or modified unless the court finds the existence of conditions that justify the reopening of a judgment. 

Modification of maintenance must be based upon the parties' needs and circumstances at the time  rather than speculation about future conditions.  The trial court is without authority to modify a decree unless the threshold requirements of changed circumstances and unconscionability are established.  The statute above makes it even more difficult to obtain modification of property division, in the interest of finality.

The party seeking a modification bears a heavy burden, intended to discourage repeated or insubstantial motions for modification.  Failure to meet the heavy burden results in denial of modification.  Also, where the alleged change in the circumstances of the parties is one that the judge anticipated and made allowance for when entering the original divorce decree, such change is not a ground for modification. 

In exercising jurisdiction to modify maintenance courts will proceed with caution, and unless the evidence clearly shows that the original decree is no longer fair and just, it likely will not be changed.