How long will the colour last on my roof tile or slate?

In common with all construction materials, roof tiles or slates gradually change in appearance over time through the action of natural weathering. these include:

The growth of lichen, algae and moss on the surface. The accumulation of fine dust deposited from the atmosphere. The chemical and mechanical action of rain and run off water. The chemical effects of prolonged exposure to sunlight.

Given the complexity of weathering and the influence of local exposure conditions such as aspect, air purity and average relative humidity, it is not possible to predict with certainty the time period over which such colour change will occur or the outcome at any particular point in time.