Remove the shells and heads of the shrimp.
Pound the shrimp heads using a mortar and mash in 3 cups of water.
Strain the shrimp liquid and set aside.
In a large casserole, bring 10 cups of water to a boil.
Cook the noodles according to package instructions or until soft.
Rinse the noodles in cold running water. Drain well and set aside in a platter.
Heat oil in a pan. Add the garlic and onion. Sauté for a few minutes.
Add shrimp, fish balls, tofu, and season with salt and pepper. Sauté until the shrimp is done. Set aside.
Put 3 cups of water in a saucepan, add annatto powder, ground pepper, fish sauce, and flour dissolved in water.
Simmer over medium-low heat. Stir constantly for about 3 minutes or until liquid starts to thicken.