If I Go to the Hairdressers and get a Semi Permanent dye will it wash out completely?

I have light blonde hair. I have only ever had foils to go blonder once. I am naturally blonde but for some reason as I am aging my hair is going browner. FROM THE ROOTS :/ If I get a 28 Washout done at the hairdressers Will I get my original light blonde colour back? Thanks :)