Apparently, there is a "blonde hair" gene different from the typical European blonde gene that gives dark skin people blonde hair. 5 to 10 percent of the Solomon Islands natives have blonde hair.
Learn more at http://www.dailymail.co.uk/sciencetech/article-2139462/Riddle-Solomon-solved-Scientists-South-Sea-islanders-blond-hair-didnt-come-Europeans-evolved-separately.html
The exposure to the sun does naturally lighten most haircolor. However, a rule of thumb, is that the lighter your hair starts out naturally, the more natural highlights the sun will bring out.
Gardening, hiking, and other extensive outdoor activities often spark natural lightening of the natural hair. If you are starting off with brunette or black, typically, off-black to darkest brown is the lightest the hair will lift naturally.
Lighter colored hair is often associated with lighter colored eyes. Thus colored contact lenses often accentuates a new haircolor change for those with darker eyes.
Photo credit on pictures: Alamy, found at website: www.dailymail.co.uk