You should never use sandpaper on your piston, it is almost impossible to get all the abrasive cleand off. As for de-greaser, grease is good. You can use hot soapy water if you wish, then oil it down immediately with 2 stroke oil.
Use a scraper to remove the carbon, but be sure you dont gouge anything.
I like credit type plastic cards for most scraping. They are tough enough to hold up and do not scratch metals.
Keep in mind that you do not need to remove all the carbon. As long as there are no obviously thick carbon deposits, some discoloration does not matter. After all, it will look that way after the first few miles anyway.