I'm guessing the joke is that she didn't actually offer her lip balm to him, but he said "it's fine" implying he was thinking about sharing.
What does the panel before that mean? "It's my color, actually."
Edit: ok I think she meant "mine is color-tinted lip balm," and he said he doesn't mind using that kind, even though she wasn't actually offering. Implying he was fantasizing about sharing.
I get it, but it's hard to explain.