If you see the comparison, you will know my hair sucks!!

I’d never noticed my hair color until my friend asked me if I dyed my hair. When I was young, it was really light brown which I didn’t like it very much. I always dyed black or blond, until recent years( these last 5 years), I’ve stopped dying my hair color; then I found out that my hair is brown(even my body hair), some is grey and white. Why is that so? Everyone in my family has black hair, so I don’t think it’s from gene. Another thing is that my son and my nephew also have a brown hair color. The most important point is it changes depends on the season..

Now, my questions are :

Why my nature black hair turning brown and grey (even my body hair)?

Why it changes everytime?

Is it because of the food, vitamin, sth??