Go neutral as basis. I’d personally avoid white unless you have an extremely birght lightfilled space ( think glass walls). I have white walls and in uk in darker rooms and shadows it just looks grey and cold. Go for a neutral light cream, something with a tint of yellow to make your walls warmer- i.e. the same colour as white would appear if in the full sun.
for flooring I’d stick with traditional colours that again are warm; oak coloured hard flooring and oak/ pale honey coloured carpets . Use vinyl in bathrooms and avoid floor tiles at this stage that cost a lot, can crack in older houses, and can be cold in bare feet . You do not want to be replacing flooring for 20 years. Grey carpets or grey tones are cold, and imho look dirty as an associate grey with dirt.
id then find 1-3 decorative items ( pictures, posters, vase, lamp stands etc) for each room- something you have or love or see and buy. Go big for items - if you’re buying poster/ pictures get as large as you can afford or space will hold. Use the colour of these items to inject colour to the room. Look at the colour wheel to be sure the colours work well. Use these colours for cushions, throws, rugs light shades, picture frames etc. these coloured items are easy and cheap to replace when you decide you want a change. Or if you love the colours you can paint walls or wall in the same scheme to inject more character. Don’t look at latest trends, they’ll date- go for what works with your things and that you like.
think about texture to- add wools, velvets, furry textures to add warmth in same colour palet . Add wood, hemp, weaves for a more scandi modern look, add satin , leather etc for a sexy look but don’t go full tarts boudoir 😉
curate the belongings you have on display. Don’t spatter gun photos, pictures, books or plants all over the room- cluster/ group items together and use odd numbers of groupings. Pinterest has loads of ideas on how to dress a room using this curation.
in terms of curtains/ blinds, settees etc- go as neutral as possible.they are really expensive and if you go for colour and fall out of love with them, it’ll be a constraint on a new colour scheme . You can “ colourise” a neutral sofa with cushions and throws. You can colourise curtains by adding a coloured header to the top - I had some cream curtains I rehashed for 25 years by adding different coloured heading strips to them to tie into my scheme- I have a sewing machine but you could do it with iron on hemming tape . Again think texture with curtains - a lovely neutral weave can add interest and warmth, silk effect will be colder but add more sophistication .
bathrooms - white sanitary and tiles. You can add texture t walls with a few mosaic type tiles, or textured white tiles. Then use towels, and bathroom accessories to add your colour - wood effect floor will give warmth to it. Again this will mean it’s cheaper and easier to sell when you move on
Kitchens - most difficult, but try to stick with neutral again and avoid too much colour. Add colour with toaster, kettle, and other stuff out on display. But if you love colour and want to,inject that - a simple single band of the tile splashback, or a couple of well placed cupboard doors can really zing.