Throw a rug over the transition zone?

The new floor will oxidize over time, turning more amber. In a couple years the match will be closer, though probably never perfect. It will also be a lot less evident once you get cabinets installed to break up the sight lines.

Weaving new flooring into old and getting a perfect match is near impossible, but if they didn't match the species, or mix of species exactly they weren't even trying. I'd try to regard it as part of the character and history of an older house-- not sure it that helps.

Using an amber-tinted sealer over all might help it appear more uniform.

Floor looks great. Those floors are 65? years old. You will never match the original wood. Looks great natural. Don't worry, and don't get it stained.

It is the light color that is difficult to match. As geoffrey_b said it is difficult to match due to age. I agree that it will age with time. I too have an older home (66 years) and chose to have the old floors refinished so the difference is not so great. I attempted to duplicate the stain by refinishing my daughter's room by myself. I stripped, sanded, and stained the floors and they are practically indistinguishable from the old floors. Of course I have not been able to walk without pain for weeks after trying to dance around with the sander I rented, but it is all good. I agree with rwiegand that you might try a sealer in a slightly darker color in an inconspicuous spot and see whether you can match it. Sealers are made in all colors and I have actually mixed colors in order to match when refinishing furniture.

Old houses don’t have to look all new to look good. It’s not going to be that noticeable to anyone else. And it’s okay if they do notice. It still looks good. It looks like what it is, but not in a bad way. And it will change over time and become more subtle.

They made a mistake with specie selection. The darker side is the orginal "white"oak floors. The new stuff is all red oak. Have them remove the red oak and redo the floors with a high quality #1 white oak.

You could have fooled me me by saying the new floor was hickory and everything has already been clear coated. If indeed they are not already clear coated and you generally like the color,try this if they will not replace it with a closer match. Use various stains to carfully blend woven boards so that the transition isn't so contrasting. Refinishing is only a hobby of mine and I have matched countless new wood parts after repairing furniture, If I can do it,anyone who hold's themselves up as a pro should be able to blindfolded. IMO professionals should know BEFORE installation and tell their customer if there will be a mis-match. At minimum they should have called time out and discussed it when new material was unpacked.

Species screw-up.

The intersection of the two floors is a great place to put down an oriental carpet...