To be honest, colours and decor are a personal matter. I mean, there are some things you ought
not to do but many more you can get away with. Take that decal on the back wall - nice sentiments obviously but not to everyone's taste - that sort of motto in my opinion ought to go in a picture frame. I see you've removed it!
The thing about tiles is you can fix them vertically, horizontally (apparently the latter is more 'modern' or 'brick bond'. Brick bond is the most fashionable at the moment but harder to do especially at corners. I'm not keen on it. If they're square you can also fit them diamond pattern (which I've done in my kitchen - but it is harder work). It is also the modern fashion to use large tiles.
My advice would be that smaller spaces need smaller tiles, so if you are tiling the areas above the work surface (I think you say you are putting up panels instead) then smaller tiles look more in keeping, but for tiling walls up to waist height maybe a 25 x 40 cm tile? Horizontal of vertical orientation, it's your choice. The largest tiles need very flat walls else it's hard lining up their edges.
Colour is the most difficult thing. Nobody ever went wrong using neutral colours, but you could perhaps use some accent colours in places to liven things up - not necessarily the tiles.
One way to try and decide on tile orientation and size is to measure up your walls that need tiling and see how, say, a 25 x 40 cm tile would fit. You can get alloy edging strips nowadays that look nice but I wouldn't put them anywhere they'd get a hard life,
For an innocuous-but-modern-looking tile but with a subtle linear pattern to it you might take a look at the BCT Linear range. Here's a link, if the mods allow links
https://www.victorianplumbing.co.uk/bct ... lsrc=aw.ds The white tile is actually a very light grey, better than it sounds - on close inspection it has a subtle striping pattern. I wouldn't go for plain white tiles in my opinion - bit too bland.
The best thing you can really do is look on t'internet at pictures of kitchens you like and try and get ideas that way. Maybe a bit of strong colour somehwere as an accent, like the splashbacks?
As a wall paint, remember to get a proper kitchen paint - which will be a silk or eggshell/sheen. I used a Laura Ashley Ivory bathroom paint (on offer) that is quite a classic colour. Not as ubiquitous as magnolia, but still a neutral. However, all I'm telling you is what I like, not what you'd like! ;0)