Unfortunately, there is no easy or simple way to do what you want.

The places that have changed color will not stain the same as the places that have not changed. The black areas will have to be lightened or any stain will not change the black color much.

Matching age and patina is done by trying colors/tints/finishes until the desired result is achieved. And true aged patina cannot be matched, since the tone is caused by time and light---so any applied color/finish will change as it ages.

And, many finishes change the stain color when applied.

Gum wood(sweet, red, black or eucalyptus) is difficult to process. It warps when drying and many sawyers prefer not to work with it. In the south is is sometimes considered junk wood, not even fit for firewood.

If you can find some red gum(West Coast especially) it might be easier to have new pieces made, since the color I see is more mahogany and red gum is reddish to start. That would make it easier to start trying to get a color close to what you have.

I've spent a couple of days trying different dyes(faster than stains) and combinations to match a piece of furniture color. I used different color shellacs along with the dyes. I got close enough to please the customer or my wife, but never did make it close enough for me.

It is simply try and try again.

I suggest using a card scraper which is faster and imo,better suited than sand paper. In all but severly damaged areas,that should reveal fresh wood. You will have to go with trial and error with stain colors since not only do different species wood turn out different colors with same stain,so does wood from different trees. Choose a high quility UV resistant clear coat first, then use ONLY stains listed on the label.

Thanks for the suggestions. I've only dabbled in refinishing some old pieces of furniture in the past, and my last project was more than a decade ago. I can see that I have some research ahead of me to find the right tools and products - fortunately this won't be tackled as a project till the spring or fall.

I've never used a card scraper, but it looks like a useful alternative to sand paper. I'm sure it looks easier on the videos though.

Gum wood was very popular as trim here in southern Ontario, although I'm not sure if I can find it new. I'll try as I'd like some scrap would to practice on before I tackle the actual sills.

Once scraped I would just give it a couple coats of varnish, which will give it an amber color. I refinished my gum trim many years ago, and I had to strip it. I will say that the window sills do take a beating and mine had a raised grain texture, looked a little better than the black color it had before.