Brown flour in butter to a light golden brown color over medium heat stirring constantly. Add yellow and green onions and cook until wilted stirring constantly. Combine juice from seafood and plain water to get 4 cups total and add to pot. Also add salt, garlic powder, paprika, and Tobasco sauce. (If you don't have Tobasco sauce, red pepper to taste will do.) Cook until gravy doesn't have a floury taste (about 15 to 20 minutes). Add crawfish or shrimp and cook another 15 minutes. Serve over hot rice as a main dish with a fresh green salad on the side.