Discoloring comes from a couple things. The first most obvious is the abuse it has taken from the elements over the years. Nearby trees may also add to its discolor or allow the roof to grow algae. In some cases this color may be your actual shingles granules (Small rocks that build up the top layer of shingle) have been lost. This is important to pay attention to as the first layer is what is able to deflect the sun. Without this the sun will tear into the shingle and potentially case premature failure in the system.