Traditionally a tiara was a type of crown, often becoming narrower at its top, and made from either leather or fabric. They tended to be heavily decorated with jewels to show wealth. A green tiara is a tiara which has the base color of green, which, traditionally meant that the fabric or leather being used was dyed green before being decorated.
While in the past they tended to be reserved for royalty, today many non-royal girls and women wear tiaras. These tiaras tend to be made from metal, and are either a closed or partially opened circle. Some are decorated with real jewels and are very valuable, while others are decorated with fake jewels. Some very inexpensive tiaras today are just a simple metal, or even plastic design with no decorations. In the modern day, a green tiara is one which has a green base, made by coloring the metal or plastic with dye before choosing whether or not to decorate it.
While once reserved for royalty and made from fabric or leather, today many people from all walks of life wear tiaras, and, much like this demographic switch, the material itself has switched as well, to either metal or plastic depending on the quality of production.