Metallic paints can be used in a number of different ways. They have been around for many centuries, but have not tended to be used for house interiors till fairly recently.
Metallic paint is produced by suspending powdered metal in an oil base. The basic idea has been around since the Bronze Age, but the modern process has improved greatly throughout history. The base almost always has to be oil as water based metallic paints are prone to rust and discolouration. Some non-rusting metals are an exception to this. Mica powder can produce good paint results in oil or other bases.
Metallic finish can be added to other paints, keeping the same general colour while altering the overall effect and lighting properties. This mixing process is a skill, and requires the powdered metal to be mixed with a thinning agent before being added to the paint. Incorrect missing methods cause clumping and air bubbles in the paint, which produces an unsatisfying result. Care must also be taken to calculate the right amount of paint; it is hard to make a second batch identical to the first, and using two different batches of paint on the wall gives an uneven finish.
The flake size of metallic paint is one aspect of its appearance. Coarse metal flakes produce a brilliant finish; very fine flakes look dull, though this is sometime the desired effect. Coating the metallic paint with a gloss will give the illusion of depth; glossy metallic paints look very thick with the addition of a good glossy coat.
There are two contrasting results aimed for with metallic paints. One is metallic paint on a rough surface, which give a pleasant antique, rustic look. The other is metallic paint on a very smooth surface, which allows a lot of reflective light, and produces a very slick, modern look.
It is rare for a room to be done entirely with metallic paint, but railings, furnishings, frames sculptures and other items can be extremely effective with a good metallic finish. Decorating a wall with a metallic mural is another good option.