To be fair you're never going to get a perfect match as application can change the colour. Listen to slammedorion as that's the way to do it. Colour up the smallest area you need and fade it out. Lacquer the bottom of the door and put the blend in the lacquer. Try and keep as small as possible to make the blend easier.

You can buy aerosol fade out thinners which if you flick a couple of thin coats over the end of the blend it should make polishing easier.