Brassicas have always been my favourite veg, right back to childhood and would definitely pass the hair colour test, I am 78 and my hairdresser tells me that my hair is roughly 40% grey. I colour it because I am not going grey evenly, white all round the edges, while the centre area is still dark brown.
However remaining dark haired into old age is a genetic feature in my father's family. My grandmother was dark haired into 70s as were a number of her children, I and a number of my cousins, have also inherited this gene for dark hair in old age, while our siblings, like my sister, 7 years my junior haven't. So whether my dark hair is caused by my cabbage eating or just genetic inheritance, I do not know