I'm inclined to say the lighter of the two, as your tiling will visually encroach into your adjoining space, beyond that breakfast bench. Because the tiles will not be installed between walls, with an open end, a dark tile would be difficult to integrate in paint schemes without dating quickly. I don't think the dark would be an issue as far as depth of colour goes, I just feel the subtlety and lightness of the soft grey is more sophisticated in a period home. While the dark grey lends itself to a more contemporary environment. If the spalshback in question was a different layout, ie stream lined or enclosed I would like the charcoal myself, but in considering your entire layout, I would go the other route.

I have just chosen my kitchen and family room colours and decided to stick to white floors, cabinetry, green glass painted white splashback which gives a lovely soft green colour and shitake ceaserstone benchtops. Once this is installed I will bring in colour with vases and accessories. Main concern is any colour you bring in will date quickly and you will get tired of it. if you keep it simple you can dress it up. It is a huge decision so chose carefully, your kitchen specialist is always a good guide, listen to their suggestions. Good luck.

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I'm sorry. It's guesswork a lot as what seems obvious to the poster isn't necessarily to the reader. It seems as if there might, might, be potential for 'go for it' to turn into 'expletive deleted this stuff is dirty/discoloured and won't peel off, how do I fix my kitchen, I'm sending my ex-friend the bill for this mess'. I mean we don't know how long you're going to be living there, whether your friend is thinking to renovate longterm, whether preserving resale value is important since the kitchen looks fine as is, whether either of you have thought that far ahead. And something like 'how to brighten' in the title would of sounded better than 'awful'. One person's too dark is another person's cosy and colourful.