Heat canola oil in a medium non-stick skillet over medium high heat. Add onion and garlic and sauté until tender, about 2 to 3 minutes; stirring frequently.
Add zucchini and yellow squash; sauté until tender, 3 to 4 minutes; stirring constantly. Remove from skillet and set aside.
Warm tortillas for 10 to 15 seconds on each side in an ungreased pan.
Assemble tacos by spreading warmed refried beans over each tortilla. Top with zucchini & yellow squash mix, and sour cream.