A glimpse of Medieval life:
14th century, Les Enluminures
How to Wash Your Hair, 12th century:
"After leaving the bath, let her adorn her hair, and first of all let her wash it with a cleanser such as this. Take ashes of burnt vine, the chaff of barley nodes, and licorice wood (so that it may the more brightly shine), and sowbread... with this cleanser let the woman wash her head. After the washing, let her leave it to dry by itself, and her hair will be golden and shimmering... If the woman wishes to have long and black hair, take a green lizard and, having removed its head and tail, cook it in common oil. Anoint the head with this oil. It makes the hair long and black."
~ The Trotula (12th century)
I've personally found the lizard conditioner to be a very effective coverup for graying hair.
And here's the updated version: How to Wash Your Head, 1612:
"You shall finde it wonderfull expedient, if you bath your head foure times in the yeare, and that with hot lee made of ashes. After which, you must cause one presently to poure two or three gallons of cold fountain water upon your head. Then let your head be dryed with cold towels. Which sodaine pouring downe of cold water, although it doth mightily terrifie you, yet nevertheles, it is very good, for therby the naturall heate is stirred within the body, baldnesse is kept backe, and the memory is quickened. In like manner, washing of hands often, doth much availe the eyesight."
From the always interesting Ask The Past.~William Vaughan, Approved Directions for Health (1612)