Hairdressers tend not to do henna as you have to leave it on for hours, and that's not realistic in a hairdressers' chair! Also do bear in mind that if you have white hairs, it will only dye them orange and they will glow. Henna is an orange hue that varies from bright to dark, but is still basically a red colour. It will turn brown hair a lovely deep reddish colour, or brighter depending on which you choose, but your white hairs will be really bright orange! Also you can't then chemical dye over the top of henna- it may turn it green (!). Plus you have to wait for it to gradually fade out. Best bet is to ask a good hairdresser for advice, as they must deal with your situation every day.