A player cannot refuse a visit from a noble.
But can you choose to use jokers to change which noble visits if you don’t have the gem colors required at end of turn?
A player cannot refuse a visit from a noble.
But can you choose to use jokers to change which noble visits if you don’t have the gem colors required at end of turn?
No, you cannot.
From the rules:
At the end of their turn, a player automatically receives the visit from a noble if that player has the amount of bonuses (and only bonuses) required.
(emphasis mine)
A visit from a noble is determined only by the bonuses printed on the development cards. Tokens, including the Joker token, cannot be used to get a visit from a noble.
Nobles visit base on mines, not gems. You need mines that produce gems that match what is listed on the noble card. What gems you have is irrelevant.