Preheat the oven to 350°F (175°C).

Melt butter in a large skillet over medium heat and sauté onion until translucent, about 5 minutes. Remove from heat and stir crabmeat, shrimp, and half of the Colby cheese into the skillet.

Fill each tortilla with a large spoonful of the filling and roll them. Arrange tortillas seam-side down in a 9x13-inch baking dish.

Mix half-and-half, sour cream, melted butter, parsley, and garlic salt in a saucepan over medium-low heat until well blended. Pour sauce over the enchiladas and sprinkle with remaining Colby cheese.