You might encounter very old fixtures that are basically designed the same as modern ones, but they have a mogul base. The socket for these are also female threaded, but they are about twice the diameter to accommodate bulbs with that mogul-style base. Replacement bulbs are still available for these fixtures. Light fixtures are sealed to be completely watertight, so the air inside reaches extreme temperatures unless it is cooled by contact with the water. Never turn these lights on if there is no water in the pool.
Most fixtures have lens overlays available in a variety of colors.
These are plastic lenses that snap over the glass lenses to make the light blue, green, or whatever color. Being very thin plastic, these too will quickly melt if the light is operated out of the water. Fixtures are held in the wall of the pool or spa in a light niche. A niche is a metal can large enough to contain the fixture, cemented watertight to the side of the pool. A waterproof conduit leads away from the niche to above the water level out of the ground to a junction box.