So, I was having the same problem. I stayed late after lab and Dr. Robson walked me through it. I learned the key to de-colorizing is to let the ethanol drip down to the corner of the slide after each drop. This way you can really monitor whether it's still purple or not. I wasn't waiting and watching it fall of the edge of the corner. This makes the distinction much clearer.
Gram Stain Mistake / Discussion
This is the discussion related to the wiki page Gram Stain Mistake.
