Bring large pot of salted water to a boil over high heat. Add the pasta and cook according to the package instructions. Drain in a colander, rinse with cold water, and set aside.
In the same pot, melt the butter over medium heat. Whisk in the flour and cook until bubbling, about 4 minutes. Add the milk, whisking to combine. Bring to a boil, whisking continuously to prevent scorching. Reduce the heat to medium-low and simmer, stirring occasionally, for 25 minutes, until the mixture has thickened.
Add the pimento, Gruyère, and Parmesan cheeses and stir until completely melted and smooth. Add the scallions, cayenne, paprika, and white pepper. Season with salt and black pepper. Add the pasta (reheat it briefly) and mix thoroughly to coat with the sauce. Serve Immediately.