Prepare the filling: Julienne the red and yellow bell peppers, slice the purple cabbage thinly, thinly slice the white onion, julienne the mango, and carrot, and shred the lettuce. Set aside.
Prepare the sauce: In a small bowl, mix together the Greek yogurt and soy sauce until well combined. If desired, add the peanut butter and mix until smooth. Set aside.
Soak the rice paper: Fill a shallow dish with warm water. Working with one rice paper wrapper at a time, dip it into the water for about 5-10 seconds until it becomes pliable. Remove the rice paper from the water and place it on a clean, flat surface.
Add the sauce: Place a tablespoon of the prepared sauce in the center of the rice paper wrapper.
Assemble the rolls: Place a small amount of each filling ingredient on top of the sauce in the center of the rice paper wrapper, leaving enough space on the sides to fold. Be careful not to overfill.
Fold the rice paper: Fold the bottom edge of the rice paper over the filling, then fold in the sides, and roll it up tightly into a cylinder, similar to wrapping a burrito. The moistened rice paper will easily adhere to itself, creating a seal.
Repeat: Continue assembling the rolls until all the filling ingredients are used up.
Serve: Serve the spring rolls immediately with the prepared dipping sauce on the side.