Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease with cooking spray.
In a large bowl, combine the grated zucchini, grated carrot, chopped spinach, and chopped red bell pepper. Stir to mix evenly.
In a separate medium bowl, whisk together the oat flour, whole wheat flour, whey protein powder, baking powder, baking soda, salt, garlic powder, and black pepper.
In another bowl, beat the eggs, then add the Greek yogurt, almond milk, and olive oil. Mix until well combined.
Pour the wet ingredients into the vegetable mixture and stir gently to combine.
Add the dry flour mixture to the wet vegetable mixture. Stir gently until just combined; do not overmix.
Fold in the grated Parmesan cheese.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 22 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.