Place the shrimp heads in a food processor and add enough water to come up to two-thirds the level of the shrimp. Blend until smooth. Strain the shrimp head mixture through a fine mesh strainer, using your hands to squeeze out all of the liquid, and then discard the solids. Return the strained liquid to the food processor. Add the shrimp bodies and puree into a smooth paste. Set aside.
Heat a large wok or large deep skillet over medium-high heat until hot. Add the oil and saute the garlic, stirring often, until golden brown. Add the annatto seeds with some of their soaking liquid and the blended shrimp paste. Sprinkle in the salt. Simmer for 10 to 15 minutes, and then add more salt to taste.
For the palabok:
Toast the flour in a saucepan until light chestnut in color. Whisk in the salt and 3 cups water until smooth. Saute the garlic in oil in a work or large skillet until golden brown. Do not overcook. Add the annatto to the flour mixture and then strain the garlic oil into the pan. Simmer over medium heat, stirring constantly, until thickened.
For the noodles:
Cook the noodles in a pot of boiling water for 7 to 10 seconds. Drain.
Divide the noodles among 4 to 6 plates. Spoon the palabok and shrimp sauce over the noodles. Garnish with chicharron, green onions and eggs. Serve with lemon wedges.