What to do with my floorboards?

[Grockle]

I just moved and ripped up the manky laminate that was downstairs. Underneath are lovely floorboards - a couple of have replaced with pine ones that don't match but I don't know what to do now. I have lots of questions:

Ideally, I'd stain/ varnish them but will I get the new, pale pine boards to look the same as the old dark ones?

Could I paint them?

How do I fix small gaps between some of the boards where the edges have worn/ deteriorated?

Could you swap the pine boards around so that they all fall in a particular place, say, under where you will put a rug or behind the sofa? We had to do this at our last place, and even though you could see the newer boards under a piece of furniture, you wouldn't notice unless it was pointed out. Basically the colour was the same, but the grain was very different. Maybe it is worth sanding two samples and seeing how each takes your stain?

Thanks - I will try that. The pine ones are all together and one set with go under the sofa. The other set could go under a rug so isn't a huge problem really. Do you know if I can fill big gaps/ small holes somehow?

When I moved into my new house I found there very worn floorboards. I hired an industrial sander and sanded as much as I can, then replaced one floorboard which appeared to be of different colour (whitish pine). Screwed everything properly and used wood filler mixed with sanded wood dust (from sanding) to patch deteriorated ends (couldn't find any other viable solution on internet, unless you want to cut out the end of deteriorated floorboard and replace with the new one). Then I varnished with 6 layers of non-coloured varnish, as this was kitchen/diner I am talking about. The new floorboard lost its colour and assimilated with the old ones, so did patches. If you are not looking closely, the floor looks OK.

However, after all I came to regret chosing to renovate my floorboards and would love to put laminate flooring on top. After almost one year, the patches look dried and worn and it looks it may crack and fall any time. I didn't check the thickness of every floorboard (only checked some), and it appeared that after sanding some most worn floorboards are only about 1.5cm and they slightly sag when a heavyweight person walks. The floorboards are not tongue-grove, and the gaps in between annoy me everytime when my child drops food and it goes inside. Slight moving of the fridge left dent marks in the varnish. Although having been screwed, the floorboards start squeeking here and there everytime. I hate cleaning the floor as I have to bend everytime to clean it properly. I can't wait to find money and put laminate flooring on top. Someone after me can just rip the laminate off and do what they want.
Mind you, I am not a professional, I am only an amateur DIY woman. But this is my (quite negative experience).
P.S. I did the same with my hallway and living room floorboards. Thankfully they were less worn, do not have any patches and look quite lovely. I woulnd't put laminate on them. So it is very important to estimate how much your floorboards are worn.

Thanks - that is so helpful Toja. I am planning on doing this in the front room & hall & poss dining room so wouldn't be used as much as the kitchen and I will make sure I check the thickness of the boards. I think they are ok for the mostpart, at least but I'll have a good look.

If you mix PVA glue with sawdust you can fill holes and gaps.

We had problems like this in our Edwardian house.I visited lots of salvage yards to find a pretty good match.Then spliced in the new[old] floorboards and then sanded and varnished.I employed a handiman to do this.We ended up using old pitch pine.Most yards will let you take a sample away for matching.I have to say unless you look really closely the new ones are pretty indistinguishable.