Can I use epoxy grout on tiled shower and bathroom floors?
A lot of people like to tell horror stories of epoxy grout and over the years its uses has become a bit cloudy. However, epoxide resin based grouts are specifically formulated to resist chemical attack and to provide a durable, abrasion resistant waterproof finish that is hygienic and impervious. 

The cured grout does not transfer taints to foodstuffs, permit the entry of bacteria, or dirt and is easily maintained in a sterile condition. With this in mind it makes a perfect food counter grout. 

As for use in a shower base it is perfectly acceptable to use and in some cases preferred over cement based grouts as it does not allow water to penetrate. 

As for staining and discoloring these will happen in a lot of situations it really depends on the quality of your water and what minerals it has in it. Over time they will tend to leech on any surface that has water coming into contact with it. Even plastic nozzles will do it. However most surface cleaners will remove such stains very easily as long as they are not left for extended periods to build up. I should also point out these stains wont appear over night. They can take months and or years so its not really something concern yourself with hugely.