Place a rack in the center of your oven and preheat to 350 degrees F.
Lightly coat a standard 12-cup muffin tin with nonstick spray.
Divide the spinach, red bell pepper, green bell pepper, and tomatoes among the cups (they will be about two-thirds of the way full).
In a large bowl or large measuring cup with a spout (my favorite because it makes the mixture easy to pour), briskly whisk together the eggs, egg whites, salt, basil, oregano, and pepper until well combined.
Carefully fill each muffin cup three-quarters of the way to the top with the egg mixture.
Sprinkle the feta evenly over the tops of the cups.
Bake for 24 to 28 minutes, until the egg muffins are set. Let cool for a few minutes, and then run a butter knife around the edges of each muffin to loosen it.
Remove them from the pan and enjoy immediately, or let cool on a wire rack and refrigerate or freeze for later.