Semi Permanant Colours~ Safe for respecting hair's condition?

Hairdressing professionals, are the semi permanant colours that last for about twenty eight washes safe for your hair? Will they damage the condition or lead to grabbing/patchy colour with repeated use? I am thinking of having my very long hair done in a salon that uses TiGi Natural Glossing hair colour. Do you know anything about this product? Is it good? will it respect the condition of long hair? My hair is darkest brown and I want to keep it that way, covering 5/10% grey hair but maintaining the good condition it is currently in. If not, can you recommens a particular brand/product that is better. I live in England so mainstream makes only, please!! Thanks for your time & assistance!