The gray ribbon is for brain tumors. Cancer in the brain is generally a tumor.
I don't know where you got the butterfly idea but it only matters to you not every one else.
I have a gray ribbon tattoo and so do several of my friends.
If you google Brain Tumor ribbons or even brain tumor ribbon tats you can see several.
Major symptoms of brain tumor are lacking coordination, weakness and inability to move on one side of the body, mood disturbances, difficulty thinking, confusion and disorientation, and wide emotional mood swings.