Invisible means that you can't see it. So, anything that is invisible has no colour!
'Have you got any invisible ink?'
'Certainly, sir. What colour would you like?'
Do you like this joke?
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