It is easiest if the boards run parallel to the opening between the spaces as it looks better to finish complete boards rather than have a line cutting across the width of a board. In my previous home, the owners at the time chose to refinish the bedrooms and install a flat oak board like a saddle underneath the doors for a break point.

If it is parallel to the boards, you can just tape off the existing floor and have the finishing done up to the tape. This requires extra care to keep the equipment from straying on to the finished floor, but it is not impossible. In that house, the dining room and hall was carpeted when the adjoining floor was refinished. When we removed the carpet, the original finish color did not match the refinished wood but it was not very noticeable.

My husband was concerned that the newly laid kitchen flooring in the current house would not match up to the original floors and the different wood cuts would absorb stain differently. So, I asked him how much the different finishes bothered him in the previous house. He had completely forgotten the two finishes did not match and those spaces were different type, sheen and color of finish. When we were staring directly at the newly finished floor, it was obvious but only stuck out for a short time. In the photo you can see the original boards at the top and the rift cut at the bottom. The darker colored board came from stain seeping under our tape line. We used the same stain but finished the floors at different times in the project.

Thanks, Lyndee! We've just been talking to the contractor and another option is to patch the hardwoods where it's needed from the new island. This option appeals to us price-wise, and they said it will barely be noticeable. Has anyone patched the flooring in a kitchen remodel? How did it work out?

My floor contractor matched red oak site, natural site-finished perfectly.

"My floor contractor matched red oak site, natural site-finished perfectly."

Regardless, it's wise to mention potential issues to customers before they happen. Managing expectations ahead of time is far, far preferable to dealing with the issue in case it does happen.

In your case it sounds like the color match should be decent. Other issues are height (there may be a slight but visible undulation where the two meet) and a slight difference in the finish (sheen and prominence of the grain).

Can you spot the replacement board? It isn't a fair question because we put the board in before the refinishing. That is an example of a successful patch. Most homeowners don't think about fixing a floor until it is time to refinish because it is expensive to replace just one or two boards. You might end up with a couple square feet of $100/sq ft flooring but that is a good approach for some situations.
Unfortunately, I don't have any photos of a poorly matched board because I don't think to take pictures of such things. We once replaced a plain sawn board with a quartersawn one from the new wood being installed in the kitchen...tight even grain pattern didn't absorb much stain so entire boatd was too light. In my previous house there was one replacement board in my son's bedroom that did not match the rest but I never noticed it until I was prepping the house for sale. Once the house has furniture and is being used, the floor issues tend to fade away into the background.

If your floor was refinished recently and you know what finish was used, ask the floor guys whether they would recommend doing a screen and coat over the entire space after the kitchen work is finished. That would not hide color issues but would give a uniform surface finish so light would reflect more evenly.