I had fun with a bleach pen with this one - it worked very quickly, so I advise doing one at a time so you can wash the bleach off the moment you want it to stop working. I found out by googling that you can reverse the possible damage by bleach (long term) by treating with Hydrogen Peroxide.
Anyway, I thought the bleaching looked a bit fuzzy, so I then added in some green permanent marker for a bit more fun