resoldering copper waste pipe
The main problem, if any, assuming the solder band is complete and without flaws or gaps is if there's too much solder on the pipe and you cannot insert it into the fitting. The pipe is "tinned" or coated with solder, and there's nothing wrong with that. 

The usual approach to making sure that the coating is thin enough is to wipe it down (while the solder is molten) with a rag - easiest if done when you heat it up to take it apart, but you can heat it up again and do that if the fitting won't seat properly as it is. That should leave it thin and nearly mirror-shiny, which is what you want. Caution is needed as you are dealing with hot, molten metal.

The basic secrets of soldering are that things should be physically clean (shiny metal) chemically clean (use appropriate flux) and not overheated (too much heat burns off the flux and solder won't wet the newly-oxidized surfaces.) The silvery solder is perfectly fine shiny metal. Dull gray solder needs to be cleaned up, but there's no need to go to copper.