Stone cills - appearance deterioration

[Lincolnshire Ian]

Hi all

We are going to have stone cills installed in our timber-framed house. My concern is that whenever I see a house with traditional buff coloured window cills they are various shades of brown, grey and black depending upon how long the algae and mould has been attacking it.

I think I have two options:

1. Coat the cills with something to prevent the buildup of crud - but I don't know what!
2. Use a dark charcoal/granite colour which won't show the algae and mould, but then I have the problem of finding a mortar to match the dark cills.

Does anyone have any ideas regarding how I can approach this?

Thanks