Cook Pasta according to package but shave off about 2 to 3 minutes. You want the pasta a little underdone because it's going to bake in the oven. Mac & cheese is not the time to choose a delicate pasta go for a hardy style so it will hold up under the cheese and sauce.
While the pasta is cooking take a 9 x 11 or deep cassrole dish a brush it with some melted butter. Makes serving easier and clean up too.
Cube the Velveeta into small cubes. Take the bread crumbs and place in a small bowl. Add 1/4 Cup of the Velveeta and the Colby mix together and add enough melted butter to moisten it this is you topping.
In a very large mixing bowl place all the cheeses, cream, eggs, and butter combine. Stir in pasta until well combined. This is the time to add salt and pepper. Remember cheese is salty so be careful. It might take a couple of tries until you get where you like it. If it seems dry add regular milk or half n half.
Pour into baking dish and cover with foil. Bake at 350 for about 30min. Remove foil and pour topping over dish place back into oven uncovered and let it bake until the cheese is melted and the crumbs are golden brown. But, keep and eye on it don't let it go to far.