A permanent color penetrates your hair cuticles inside the hair shaft, reacting to form large color molecules that are trapped in the hair shaft until you re-color it, let it grow out or cut it off. It will normally contain ammonia or peroxide and can cover greys.
A hair gloss adds a boost of color by depositing pigments that sit on top of your hair cuticle, not inside the shaft. It adds shine, refreshes your color – natural or not – and nourishes your hair so it looks and feels super soft, healthy and glossy, although it's not designed to cover greys. No ammonia or peroxide required.