There is one thing that would make me nervous if this ends up in court. If the lease indicates that 1st and last months rent is part of what is required, they might be able to successfully argue that the lease was never fully executed because those amounts were never collected and they never moved in. It might be worth a call to an attorney to get an opinion. Keep in mind Bill S.'s comments from above.
"Finally, you can not just "keep" the Security Deposit. You must send them an accounting of how it is used no matter what. Colorado law allows for 30 days to return the SD unless the lease specifies otherwise, in which case the law allows for up to 60 days to return it.
If you do not send them the accounting, the court can order you to pay triple the SD deposit back to them even if they owed you the whole amount. Don't for get to send the accounting in a timely manner!".
The last thing you want to do is end up owing them more than the rent they owe you and you are fast approaching the date where that could happen if you haven't followed the law and they take you to court. If that happens and they win you could also be on the hook for their legal fees. And there are lawyers that will take that on a contingency fee arrangement, and they will charge somewhere in the $250-$500 per hour range. Given that you are arguing over a few hundred dollars this could go south rather quickly.