Have you heard anyone talking about exclusively using distilled water for watercolor painting? If you spray your palettes and have experienced premature molding, you may be a thinking you could be a victim of tap water!
It is true, some tap waters contain minerals that may contaminate watercolor paints. Although it is rumored tap water causes mold, the primary cause of mold in watercolor paints is storing them in wet and moist environments. (Be sure to allow your palette to fully dry before storing them). When it comes to the preservation of watercolor paint, there is no difference between using distilled and tap water.
I do feel it is very important to note that distilled water is slightly more acidic than tap water, so be cautious when using archival paper and distilled water.