Most likely, the dark green/black roof stains robbing your home of curb appeal are most pronounced on the North and East sides of your asphalt shingles. It's not dirt, and it's not random; your asphalt shingles are loaded with crushed limestone filler which is a nutrient source for Gloeocapsa Magma, a hardy green-blue form of roof algae that uses the nutrients in your shingles, moisture from rain, and sunlight to grow and spread.
The roof algae, often called roof mold, is spread by wind and animals, and can't be prevented. But luckily roof mold can be removed! You don't need to replace your roof if roof stains just appeared, but you do need to clean the shingles. Count on the professionals at The Roof Cleaner for a deep, lasting roof cleaning.