Di-ionised water is cleaning the roof tiles!

Hi New to the forum but have been in the business for 10 years.  Customer asked why there is discolouring on the roof tiles where the water runs off the windows and on to roof tiles. Never noticed in 10 years,  I looked and agree that the water seems to be cleaning the tiles and leaving them a different  colour to the rest of the roof.  I then started looking at other customer windows which allows water to run down on to a roof.  Again all discloured due to the fact that they have been cleaned by the dionised water.  As anyone  else had this.  Its great that its keeping the tiles clean, but not in a way that the tiles are a completely different t colour to the rest of the roof. Any experience of similar?

Pure draws out anything it touches. Every time it lands on the tiles it draws out some of the dye that's in the tile. It will eventually leave the tile like a sand and cement tile if that makes sense. Are they slowly turning light grey?