Almanac Asian Am. Scientists think the red color is caused by impurities in the planet's atmosphere constantly absorbing ultraviolet and violet light, such as sulfur or phosphorus. The Great Red Spot is quite a mystery.
Nobody is sure how long the storm has been going on.
All we know is that the spot has been visible since the day we first built a telescope strong enough to see Jupiter in the 17th century. Here is a link to pictures of the Great Red Spot taken by Voyager I in 1979. You can learn more about our solar system at our Astronomy channel.