What is "semi-permanent" hair color? How long does it last?

Semi-permanent color is ideal for trying a new hair color shade, and lasts through four to six shampoos. Used straight from the bottle, it contains no ammonia or peroxide, so it is mild enough to be used on fragile, permed or relaxed hair. It blends or covers gray (if hair is less than 25% gray); adds richness to natural hair color; gives highlights and shine, but does not lighten hair color.