"Certainly, sir. What colour would you like?"
'Certainly, sir. What colour would you like?'
Invisible means that you can't see it. So, anything that is invisible has no colour!
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