This is not as unusual as you would imagine. Babies are often born with unexpected hair and skin colour combinations. Often the colour of hair and skin that they are born with bears little resemblance to the colouring they will have as a child and subsequently as an adult.
More often the hair will be a different colour at the front and back of the head. It’s all to do with pigmentation and is more noticeable in children of mixed race. The pigmentation in our hair and skin can alter throughout our lives. A person born with very blonde hair can be dark in later life. This streaking can also be seen as people get older and begin to turn grey. Turning grey is simply a loss of pigmentation in our hair.