Strapless leotards can be worn as undergarments or as dance wear. Because they are form fitting, they let you move freely, unencumbered by extra bunched up fabric. If you do opt for the strapless type of leotard and you intend to wear it for dancing, or any activity that requires a lot of movement, you should make sure that your leotard will stay up, otherwise you may find yourself constantly pulling at it to bring it back into its proper position, which can be annoying, and ends up defeating the purpose of what should be a carefree garment.
I couldn’t find many models of these, but here are three by American Apparel, The mint one being my favorite:
Green strapless leotard pictured: American Apparel Cotton Spandex Jersey Strapless Ruched Bodysuit in Lucky Green T, Lucky Evergreen, and Lucky Mint
As for color, as pictured above, there are several shades of green available, and you should pick one depending on the color of the other clothes and accessories you will be wearing, as well as the color of your eyes and your skin tone. These are rarely worn on their own.