I'm no expert on kitten tongue color, either (although I've been licked by a few kitten tongues ), but Lily's doesn't look "off" to me. She looks just precious, doesn't she?

Maybe you could explain that before you take her home you'd just like to have your vet do a regular checkup and that it's a routine thing. The idea of a checkup isn't that you think she might be sick--it's to help you do everything you can to keep her well.