Hi Dave and welcome to Tim's forum

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Well, as I've said before, colour schemes are very much down to personal taste and the type of look you're after; there aren't really any rules. Just try to mix the tones of your colours. Imagine your kitchen in a black and white photo. Would it be quite interesting with a lot of different shades of grey ... or would it all be the same boring tone of mid-grey?
You've made a very good start with your choice of white gloss doors with blocked walnut! Very classy.
Personally, I like a mixture of finishes and textures, so I would tend to go for something textured for the floor, with a blend of beigy browns. Paler than the walnut but dark enough (or mottled enough) that marks won't show up too much and to provide a contrast with the white of the units.
Ceramic (or porcelain) tiles which are pretending to be stone would look great and they will vary from very mottled, rustic looking ones to very straight cut, slightly mottled, flat ones. The straighter and flatter they are, the more modern they will look. I personally like diagonally laid tiles in a galley kitchen - but this doesn't look as modern as straight ones.
Don't be afraid of largish tiles just because the space is narrow. They can make it look more spacious because you get fewer grout lines.
If you don't want to lay ceramic tiles you could have a look at some of the good quality vinyls like Karndean or Amtico or Polyflor (they all do a range of prices and there may well be other makes which are good too).
If you think sticking to beiges and browns is a bit boring, you could always introduce some greys or bluey greys or greeny greys or mauvy greys into the mix and then pick out some darker shades of those colours in pots and pans or in wall tiles (if you're having any - or are you having upstands on your walnut top?). If your appliances are steel then soft greys can look good, as well as or instead of, beigy browns.
Hope that helps.