Subway tile itself is easy to keep clean, and I would suggest a darker grout in a shower, not white. Perhaps a silver, like this?

Have the grout color sealed, squeegee the tile after every shower, never use any ammonia based cleaners on the tile or grout, do not use bar soap and you can have a low maintenance beautiful tile shower for years to come.

I have white subway tile in my kitchen and in a guest bathroom shower. Both with white grout. I absolutely love it. I just spritz it down every so often with a bathroom cleaner with bleach. I will tell you after 5 years so far, it looks like new. Putting a colored grout will show any imperfections more. To me it also can look dirty grouted dark. White subway-white grout period! Sorry if I sound pushy;)