My hair does that, too - especially if I spend a lot of time swimming in salt water and then in the sun! I'll get very clear blonde streaks around my face, but it also has a reddish hue to it, too...so going blonde by boxed dye is a big no-no because it comes out orange. My winter color is definitely from my mom's side, but it's ability to change during the summer is my dad's side all the way. They've all got dirty blonde hair with natural streaks. My boys are leaning towards a dark dirty blonde/light brown so far.I've been paying attention more to that lately.
My hair color, for instance, I always thought was my mom's. It's a very distinctive brunette. I have natural blonde in my hair. It's very apparent after I've been in Florida or to the beach. During the winter it darkens. I've actually had to explain to my black friends and Asian friends that no, I don't dye my hair, it's just multicolored on its own. Anyway, I thought I inherited it from Mom until I saw a younger picture of her and realized her hair was much darker and basically dark brown without highlights. Then two of my cousins on Dad's side randomly inherited it. I asked my grandma and she said my grandfather's sisters had my color hair. My dad and grandfather had much darker.
So now when people say my mom and I have the same hair, I'm like, "Uh, no, that's not her natural color."