I recently installed Common Grade 1 red oak hardwood in my bedroom after taking out the carpet. My home is 40 years old and the existing wood has installed I assume when the house was built. Leftovers were left in the attic and I took them to a flooring specialist to match it. This was a DIY project. As I started sanding the old floor I realized they don't match.The old wood is much creamier. I'm having difficulty getting all the stain out of the old floor. Will continuous sanding of the old floor make them match? Is there a way to stain only the new one so they will match?

Yes, more sanding of the old floor will get the old stain off in most cases. I don't know what equipment or grit paper you are using, but a drum sander with at least 36grit should do the trick.

Once that's done, you're left with the challenge of the mis-match to the new flooring, right?

Did the leftovers from the attic have stain / sealant applied or were they unfinished?

I was using a random orbital floor sander and started at 20 grit. The leftovers in the attic were unfinished and definitely look exactly like the new flooring. That is why I assumed they would all be cohesive.

New flooring will be brighter looking and more pink. It will mellow over time as is is exposed to light. The flooring w ill become amber looking. The red pigment in the wood is light sensitive.