Add the curry paste, 2 tablespoons for a mild spiciness and 3 for more heat, and stir until darkened in color, 2 to 3 minutes. Pour in the tomato sauce, then fill the can a third of the way up with water to swish out the rest of the sauce into the pot. Stir, bring to a simmer, then reduce the heat to low and simmer while the pasta cooks.