I am probably 50% gray (at 36, sigh) and suffered terribly dry hair - and corresponding dandruff - and tried pretty much everything under the sun: expensive haircuts, all kinds of shampoos and conditioners, playing with the washing frequency, etc. Know what finally worked? Going retro.
I shampoo every day, use hair tonic, a dab of brylcreem, and a dab of murray's pomade. Dandruff is gone, crunchy/dry hair is too. Best of all, shit is dirt cheap and can be found at CVS.