The UV-rays in sunlight always makes the hair paler/brighter no matter if you are blonde or brunette.
That's why the roots of her hair is darker than the rest of the hair, because the roots has not been exposed to the sun yet.
Since Caro is a pro tennis player, she doesn't experience much winter, as the tennis seasons is planned so they play in Australia and other places south of equator during their summer or close to equator where it's warm and sunny year round, while it's winter north of equator.
The summer in Denmark is quite short but with long days, so here it's easy to tell how peoples hair pales during the summer and gets darker again during the winter unless they use tanning beds. If you spend most of the time outside in the sun, year round like pro tennis players does, then your hair color won't change much during the year. But your roots will be darker than the rest of the hair until it grow out and gets exposed to the sun. It can look almost like when someone who has been dying her hair a while ago and the hair has grown since, so the roots haven't been dyed.
You can get hair products with UV-protection, just like skin protection.
Hair can also get darker as you grow older. Many people who are born very blone (almost white hair) can gradually get darker hair while they grow up. So if you compare a picture of a 20-year old with a picture of the same person as an under 10-year old, he/she might have much darker hair now than as a kid.