Depending on what the shingle material and weather are like in your area, these circumstances can be rather common. The discoloration is usually due to algae growth and can easily be confused with granule loss, damage, or some other sign of pre-mature roof failure. This is mostly an aesthetic concern and can be removed with proper materials and practice. Algae growth should not affect the life of the roof unless it is layered in extreme amounts and / or covering damaged areas.