Like Trip mentioned, in Sims with grandparents around (in the same town, either living or as a ghost) there is a high chance of them inheriting the grandparents hair color. In fact, my heirs inherit a grandparent's hair color more often than a parent's. But again, results can be mixed if more than one of the parents/grandparents share the same or a similar color.
When I was playing in Isla Paradiso, my blonde heiress married a young man with aqua/turquoise hair, Desmond Torres. Desmond's father, Matteo Torres, had dark hair and his mother, Karlie Goldberg, was a blonde!
https://sims.fandom.com/wiki/Matteo_Torres https://sims.fandom.com/wiki/Karlie_GoldbergUpon researching Matteo Torres I discovered that his parents did 'live' in town, though both as ghosts who had drowned, hence the blue-toned hair color. The drowned grandparent ghosts also had blue eyes and light blue skin, but both of Matteo's children only received the aqua/turquoise hair, and had their parents skin tones, not the blue.
https://sims.fandom.com/wiki/Garcia_Torreshttps://sims.fandom.com/wiki/Florinda_TorresI use the NRaas Story Progression mod in place of the games own story progression and several of it's add-on modules for fine tuning more of what happens in the game, including genetics, to a degree.
I believe I have my inheritance from grandparents set at 25%, or maybe even lower, and have enabled a more 'blended' genetics to be the norm in my game rather than the children inheriting their genetics from one or the other of their parents.
Even so, I have children inherit the grandparents hair color what seems like more often than not.
Carly and Desmond had three children, none of whom received his aqua/turquoise hair, but their granddaughter, Lorelei, whose parents had red and blonde hair, ended up being my first heir with that distinctive aqua/turquoise color! And then Lorelei passed it on to one of her four children, a daughter, Lucinda. None of Lucinda's four children received it, though she and her mother both possessed it. Yet all three of Lorelei's other children, spares, each had a child with the aqua/turquoise locks their grandmother had.