Yellow squash tends to have a thicker, waxier skin. It’s also a bit more bulbous at the bottom, especially the crookneck varieties.
Green squash (zucchini) is usually more cylindrical and uniform. Its skin is thinner and more tender, which means you can eat it raw without feeling like you’re chewing on a candle.
Important note: Don’t peel either of them! That’s where most of the vitamins and flavor live. Plus, that yellow skin is where the sunshine flavor hides. (Yes, I said sunshine flavor. Roll with it.)