I would personally use "greying" (grey - definition of grey by Macmillan Dictionary).
Is there any single word meaning the same as 'Pepper and salt' when it describes 'hair'? Is this phrase used in everyday English?
(How about 'grizzled'? Would it mean the same?)
It does mean that some of the hair is greying, but "salt and pepper hair", for me at least, means that there is a real mixture of dark and light hair. It wouldn't really work as a description for someone who was blond(e) and is going grey. The idea is the salt is the white/grey hair and the pepper is the black/brown hair. Both need to appear in roughly equal amounts.
We don't usually use "grizzled" for hair, but we do use it for beards. I have no idea why.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.