Most brick products fade or can become discoloured, stained with soot or algae etc unless specialised treatment of a glazed finish.
I have no knowledge of all that has occurred, but Wienerburger DO have UK factories, so whatever bricks are chosen they may actually be produced in the UK - at least I expect Wienerburger, irrespective of their Head Office location, to be aware of the sensitivity about a WW2 RAF Memorial.
Was the reasoning behind the "hand made bricks" approach so that donors could "Buy a Brick" and have some message inscribed on it?
I'm not trying to score points or belittle the efforts involved, but asking questions that could resolve the potential problem points.
and Good Luck with progress towards the Memorial.