In searching the database for Colorado laws concerning renter and tenant laws there is nothing preventing a Landlord from asking any amount rent or security deposit. The owner of the property can ask for six months rent and a security deposit. He can also state in the lease no pets, no smoking, no children or pretty much any restrictions he wishes on his own property. This will severely limit the number of people who will rent the property, but it is not illegal to try.
Because we live in a free market system of government, if you own a service or item that someone else wants, you can sell or rent that for any price the market can pay. If a landlord is requiring first and last month's rent and a security deposit, he/she is trying to be very selective about who moves into this property. Not everyone has this type of cash, but some would pay this easily depending on their income. I could certainly afford this, but if I chose to do it would depend on the house or apartment as well as location and other criteria I would be looking for.
"COLORADO: There is a three day requirement on non-payment of rent notices. Colorado law provides that in certain situations a landlord may have a lien on a tenant's PERSONAL PROPERTY for rent the tenant owes the landlord."