Preheat oven to 400 degrees, and prepare a 1-quart baking dish with cooking spray.
Cook cauliflower in a pot of salted, boiling water for 5-6 minutes.
Drain and place into the prepared baking dish. Season with a pinch of salt and ground pepper.
To prepare sauce, melt butter in a small saucepan over medium-high heat. Whisk flour in so that it forms a light roux.
Gradually add milk, whisking so that the roux blends in completely. Continue cooking and whisking until sauce thickens. Do not boil. Add salt and pepper.
Stir in Gruyere and Parmesan cheeses and stir until cheeses are melted. Pour sauce over cauliflower.
Sprinkle Colby/Cheddar cheese over the cauliflower.
In a small bowl, combine melted butter and bread crumbs. Mix until thoroughly combined with a fork.
Sprinkle bread crumbs entirely over cauliflower.
Bake for 15-20 minutes, until bubbly and golden brown.