From the Beauty Team at Fiorio:

The best way to make sure your hair does not go ashy is not to over highlight. Using too much lightener will make your hair look dry and damaged and it will have no tone. Hair that has been coloured one tone will have to be shifted away from the ash by using warm tones to brighten. Special treatments can be used to remove unwanted tones. Also using semi-permanent colours helps us to stay away from ash colours.