Traditional French toast gets brown due to both the butter and the fat in the egg custard. If you use whole milk and/or half-and-half, you'll get a nice browning naturally. But the exact browning will depend on the type of milk, the type of bread, and the strength of your air fryer. Even if your air fryer French toast doesn't brown, it'll still taste crunchy on the outside, soft on the inside, and delicious!