Watch to see if the yellow margin becomes white in a few weeks. Might be 'First Frost'. But I'm just guessing as it's hard to tell how large it is from the photo. I would suggest that you take regular pictures of it and note how many vein pairs. Also keep track of when it blooms and what the flowers look like. Many hostas change colour after they emerge from what they were very early in the season. Also, if you've just bought it, it might look quite a lot different when it emerges in your garden next year.
