Note in some rare cases tap water may be very soft and cause shell erosion. Only way to know for sure is to test your water. The only other thing you can do its to got to your water utility web page and look up the water quality test report. It should list water hardness as well as many other parameters. RO and distilled water can be used if it is remineralized first. If you can use your tap water, use it. RO and distilled water is the better choice only if your water is too hard, toxic, very high or low ph, or has high levels nitrate or phosphate.
As to your snail the damaged shell will probably never get its color back. However any new shell growth should have color.