"Certainly, sir. What colour would you like?"
Q: "Have you got any invisible ink?"
A: "Certainly, sir. What colour would you like?"
The man wants to buy invisible ink. 'Invisible' means that you can't see it, so it has no colour.
The shop assistant asks him what colour he wants - but that's impossible, as it's invisible.
I hope that helps.
LearnEnglish Kids team