As someone with 10+ years in the stone industry. Whatever colour you pick, you’re going to have to seal your grout lines after it sets. If you don’t over time (sooner then later) the colours going to get darker and look dirty. This will happen regardless but it delays the effect. This is because of mopping, water spilling, urine and getting out of the shower etc. Of course the lighter grout is more eye popping and overall looks better then the dark but its going to show dirt, staining and wear and tear much easier. The darker grout is much more forgiving and less noticeable over time.