Put hydrogen peroxide in a clean spray bottle. Spray it directly onto the grout. Try not to spray too much. Use a nail brush or a small kitchen scrub brush and lightly scrub the grout. It works best with minimal pressure. On high traffic areas that have more dirt or spots that have a stain you can mix hydrogen peroxide and baking soda in a small bowl to form a paste. Use the paste with a toothbrush to scrub the hard spots. Its non toxic and you probably already have all the items in your house. It sounds labor intensive but once you get going you can do large areas at a time. The results are amazing!

Many of the subway tiles come with built in spacers for a small grout line - though you have to do some math to check that you don't have a gap at the top by the upper cabinets and fudge each tile a bit (a very little bit) to make it come out even. Mine have about 1/16 to 1/8. The grout line looks a little bigger after it is grouted. Here is mine: https://www.houzz.com/photos/my-pics-work-in-progress-phvw-vp~57088087 

Thank you! I will look into Prism as an option. Do you think urethane grout is worth the extra expense? I'm not sure if my tile installer is trying to upsell me, or if it really is more stain resistant. The reviews of the Custom urethane grout (Fusion Pro) aren't great. Lots of people that have problems with it.

I just put in a glass tile backsplash, and used Laticrete Permacolor Select NS (for non-sanded). It is a 2-part grout - you buy the base, and the color. It does not need to be sealed, and is easy to use, but it sets up fast so if you have a large area I would recommend doing small batches. https://laticrete.com/en/our-products/tile-and-stone-installation/grouts/cementitious-grouts/permacolor-select-ns-grout-base Do your research on grout for glass - the sanded grout I first tried, scratched the sample board that I made. The Mapei site does not recommend Karapoxy for glass as it is 50-75% silica sand. This is from their website. Some glass tiles are more sensitive to scratching, such as glass tiles that have soft glass and glass tiles that have an iridescent finish. These delicate glass tiles should only be grouted with an unsanded grout like MAPEI Keracolor U. Adding MAPEI Grout Maximizer to Keracolor U will help to reduce the porosity of the grout and help to keep the grout clean. Other glass tile may work just fine with Ultracolor Plus FA or Flexcolor CQ, grouting a test area first and evaluating it for scratches is the best approach when dealing with glass.