Can tap water affect hair color?

no i dont think so i think that the water that comes out of the facet is already tap so i dont think it affects it.

Yes, there are chemicals in tap water than can fade hair color. It is very important to wash the hair with color safe shampoo and use cool (or as cool as you can stand) water when shampooing.

Tap water can also cause brassiness in blonde color. This is where a purple shampoo will come in handy.