Little history: Pulled carpet from a room that had continuous carpet installed for well over 15 years. When we pulled it, we looked at the hardwood and thought it would be great to keep the hardwood.
There was a section of floor that had been removed for a floor vent, but the vent had been removed and floor replaced, with a quick/bad patch job done. There was another section that had been cut through with a saw, then had the boards just put back down. There was another board missing from near the wall where there had been a vent.
I removed the old boards, replaced them with brand new boards, and sanded the whole thing multiple times. I know that whatever seal/stain i put down will not cover and magically make them match, so I'm wanting to know if there is any way that I can bring my new boards some color, trying to get them the same starting color as the adjacent ones.
Also, have a lovely couple stains that after sanding, look slightly lighter, but still very visible.

Here's pics of the floor after sanding, showing the mismatch of color and the stain.

I don't see much sanding in the second set of pictures. You need to get more aggressive.

That's what I get for being an inexperienced homeowner. The only experience I've had with hardwood floors is either having carpet re-installed on them, or walking on them.

I rented a orbital sander, but may go get a drum sander. Problem is that my floor is wavy enough, that I'm not sure a drum will get what I need, but we'll see.

Once you get down to bare wood the old and new wood will come much closer to a match. Use an edger or diagonally sand the patched areas to get over-wood smooth before going with the grain.