This is an educated guess rather than a fully sourced answer.
One of the distinguishing characteristics of grass in cemeteries particularly, is that every time they have to dig a grave they have to strip the turf. The underside of turf is black(ish) with wiggly pale strands of the grass's roots visible, which could be seen as analogous to dark hair with grey strands, particularly as grey hairs tend to be a bit more wayward than their black forerunners.
Thus the allusion would be to the side of the turf that faces into the grave rather than its outward expression of green blades.
The association of greying hair with ageing and the approach of death may be enough to make a grave-preparation link between greying hair and the corpse's side of the cemetery grass.
I'm not sure if this tells us anything additional about Adele, though it may tell us something about the relative cheeriness of the disposition of people in the town if the slightest greying of hair leads them to thoughts of the grave, or just that they are quite observant about turf.
Edit: Having read a brief summary of the story, it may also foreshadow Adele's preoccupation with her grave-clothes.