Cook macaroni according to the package directions and drain.
In a saucepan, melt 2 tbsp. of the butter over medium heat. Stir in enough flour to make a roux. Add milk to roux slowly, stirring constantly. Cook over low heat until thickened.
Remove from heat and stir in the cheeses and Salsa Roja. Put macaroni and bacon in large casserole dish, and pour sauce over. Stir well.
Melt 2 tbsp. butter or margarine in a skillet over medium heat.
Add breadcrumbs and brown.
Spread over the macaroni and cheese to cover. Sprinkle with a little paprika. Bake at 400F until hot and bubbly. Pull out of oven and serve.