Quote Originally Posted by rockzmom
Peter: Do you want some coffee?
Kate: "Thanks, that's okay." or "Thanks, but I'm good." or "No, not right now."

In the first two, Kate is saying "thank you" which would seem like yes, but she is actually saying no. In the last one, she is saying "no" yet leaving the door open to maybe have some later. BTW, in the second one, the word "but" is often left out when actually speaking.

Let me know if I missed your point here.
In your 1. and 2. variants Kate answers about coffe at all, not about her desire (wish) of drinking that coffee. It is a reason of her "No" in this variants, isn't it? In 3 variant she replays 'Maybe I will drink coffe later'. It is halfway between "No" and "Yes"