Without a screen shot or even an attempt at describing how your floor and background "don't match", we would just be wildly guessing. Much more info is needed.
First, you needed to shoot on a green stage, not just against a screen.
Second, creating a large solid layer that resembles your floor should not be difficult but you have provided no information about what you have attempted so far. You should be able to simply sample the white of the floor for the background plate, add some texture maybe. Easiest solution might be to go back to the studio, turn on the lights, climb up on a ladder and shoot the floor
I shot a music video with a green screen and a white floor....I'm keying out the green to replace it with white...the problem is the floor and the background doesn't match...
I hate to say it, but you went about this totally bass-ackwards. You shot your video, and took it as an article of faith that AE could work miracles. You didn't run tests to see if your idea was sound. And now you're wondering why it didn't work.
It didn't work because you put very little thought or effort into making sure it would work as you hoped.
How can I make the floor match the background...
Here's the most baffling of the questions you pose. You want to match the floor -- something you can't easily change -- to the background, which is just about the only thing in this project you CAN change. Bass-ackwards thinking once again. Match the background to the floor instead.
I'll also assume that there is no way you can shoot this footage over again, keeping in mind Bogie's recommendation and Michael's inference that you should ask questions BEFORE you record video.
You're stuck with what you've got. Suck it up and make the best of it. I hope you were able to pull an acceptable key.
You can try using multiple key passes (ie apply multiple and set Keylight) combined with Rotoscope and masks.
That would be kind of a standard approach for the situaion..
If you have CS5 ...Rotobrush maybe your very best friend at the moment.