Even if it’s white tile*

I've used Spectralock epoxy grout with good results, on both floors and in showers. However it was the 2-part stuff you mix together, not the Spectralock 1 that's pre-mixed. So I can't help with that question.

On the color, I always shoot for a close match if single-colored tile, or something in the middle of the color range, if variegated. That results in grout color blending in rather than standing out, which is my preferred design style.

Epoxy grout will discolor a bit over time, but then a scrub with a toothbrush and bathroom cleaner will clean it up quickly. So no need to "build in" the dirt color in your choice.

SL1 is good stuff. What "mixed reviews" are you talking about? Read the datasheet for uses, but essentially useable for anything except the toughest applications.

Im a repeat Laticrete offender. For some time now. Id suggest letting the tile profesional on the job make the call and supply....the grout.... labor and Warranty.. pic a color 😃

Yes, as Mint says, BUT make sure you have a qualified installer with current knowledge/experience. If they do not know about the various current grout technologies, that may be a clue. Spec 1 is generally a good choice with porcelain. Color match to tile please (or mid range of tile colors).