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I spoke to my tile guy. I hadn't realised you were talking about external work.
In short his response can be summed up as htfu.
Whatever you go for will be so changed by the elements that it's not going to make a huge amount of difference.
Choose a dark grout / filler if that's the look you want and make sure it has the elastic properties you need. His opinion was more along the lines of getting the right stuff for the job rather than what it looks like as that will most definitely change. So don't stress on it.
The guy who did my tiling (and who is currently tiling my hallway) had a pack of grout samples to show me when he asked me to choose what I wanted.
If you're doing it yourself I would assume similar are available online no?
Otherwise I can ask him on Monday which grout he used that he had the sample range for