Choose your furnishings before your paint, but unless you are retiling, you need to work with your kitchen tiles. A colour coming back into fashion is terracotta, also is dark green. Any green is in fashion now, mostly the olives.

You need to find a green, then, if that's your choice, that goes with these

plus your new furnishings, and you can use it in different strengths in the different areas.

Yes we have a similar fireplace we want to replace when we can afford it. We are going for one the age of the house 1920s. I think you could put in a very lovely wood burner with a plain surround with that large space.

It's a great space and the cupboards are lovely. However, the floor colour and the paint colour, as well as the fireplace, are 'sucking' the light (and life) out of the room!! A nice light green on one wall, and pick up a chalky white or ivory for the other walls - including the fireplace - would do wonders for this space, and throw in the odd artwork/cushion to pick up the shade of orange/red in the tiles would inject 'life'. Then I would go for a light coloured flooring.♡

Aqua might be a nice wall colour too. I think your kitchen floor tiles are nice. White in the sitting room would be good too and I would ditch the brown carpet in favour of a pale grey or else some natural sisal or similar, some houseplants and colourful artwork. I'd also replace or recover (even with slip covers) the sitting room suite with a grey or taupe cotton or similar and add a couple of zingy, colourful scatter cushions. If you do all this, it may not be necessary to paint the fireplace though, by all means do this if you want. Just be aware that once you paint brick, you can't really get the paint off i.e. it's virtually impossible to bring it back to its natural colour. As this is modern brick and only a fireplace that's not so much of an issue though as it might be in a period house with old brick.

Im echoing houonocs comments- a few well placed covers or throws on the couch in an olive-taupe colour would look great, and if you didnt want to go too mad, you could always use accessories to add colour. A jewel coloured (maybe a medium dark teal?) paint on the walls would brighten up the kitchen and would be nice with the slate grey of the tiles?