Provide more details regarding what your expectations are for 'dirty' hair. Greasiness? Literal dry dirt particles? Dandruff flakes? All of the above? I mean, are we imagining: "I didn't wash my hair yesterday." or "I slept in a pile of dry leaves and manure for a week."
My initial thought is that 'dirty' hair is largely a contrast to 'clean' hair. And 'clean' hair tends toward fluffy, groomed, and shiny. I imagine you'll want to evoke the appearance of the opposite; limp, grungy, messy.
Also, where 'clean' hair has a glossy shine, 'dirty' hair has more of a greasy shine evidenced by clumpiness, hair tresses actually sticking together.