you will not get the coverage with a demi hair color that you do with a permanant color---it will also fade faster--but there will be some permanant staining especially on the grey hair--so is about as hard to grow out---I am a retired hairstylist and have been a "natural' redhead sindce I was 17 and have no desire at 76 to be anything else---I do use the permanant type of color---if the 'demi' does last 28 day you will still have 'roots'---hair grows an aveage of 1/2 inch a month---there really is not a 'temporary' color that last 28 days--it last one shampooo---that's why iit is called temporary---I would talk to my hairstylist --explain what you want to do and he-or she should be able to help
Good luck!!!!