I just ordered nano luggage in kohl. I too was considering between kohl, prussian blue and slate (I think slate is not available at the moment?). I went with kohl since I need greyish bag in my collection so I think the blue-grey shade will fill the void and I do have some blue bags already.
Since you have the same style in black, I would go for the prussian blue for a nice pop of colour. On a side note, I think the website picture tend to look a lot lighter than the colour in real life though.