Being more difficult to identify, females are generally identified by their size, conformation and coloring, because they lack the blue of the males. More information on the various species can be found in my companion article Parrotlets in Aviculture - The Various Species . Whatever material you decide upon, make sure it is easy to clean and free from zinc and lead. Also, do not use a cage made of brass or copper. Paint should be baked on to keep it from flaking.
Powder coating is the best, however it does increase the cost of the cage.
As previously mentioned, parrotlets are very physically active parrots and need a good size cage to keep them happy and healthy. It's always best to get the largest cage you can afford. This will allow the placement of lots of perches to facilitate climbing and space for a wide variety of toys in which to play. This is the minimum recommended size cage for a single bird. It is better to have a cage that is wider or deeper than it is tall.