Ha, I was thinking about that too lately.
If they ask me "Hey did you dye your hair pink/purple" again I'll tell them, no I just dyed my roots brown.
Last week a guy on a motorcycle asked me what I used to colour my hair while he was driving past. I wonder if he expected me to explain in the 2 seconds it took him to drive by me. I just told him I use watercolour. >_>
PS. I don't know if it happens to you too, but most people that ask me if I've dyed my hair can't take my word for it and take their permission (obviously mine isn't needed) and start tugging and stroking my hair to see that it's real dyed hair. I've had people I've seen a couple of times and even junkies and beggars do that. >_>