In the food processor, grind the almonds and icing sugar until they form a fine powder, 2 to 3 minutes. Sift and set aside.
Using an electric mixer, beat the egg whites with the food colouring until soft peaks form. Gradually add the sugar, beating constantly until the peaks are stiff. Using
a spatula, fold in the almond–sugar mixture and stir until the mixture deflates and softens slightly.
Using a pastry bag fitted with a 1.5-cm (1/2-inch) plain tip, pipe 2.5-cm (1-inch) macaroons onto the cookie sheets, spaced about 5 cm (2 inches) apart. The macaroons should flatten slightly; if they do not, the batter has not been stirred enough. Let dry at room temperature until the macarons develop a thin crust, about 90 minutes.
With the rack in the middle position, preheat the oven to 180°C (350°F).
Bake, one sheet at a time, with the oven door propped open with a wooden spoon, 12 to 15 minutes. The macarons should not brown.
Let cool completely. Spread a thin layer of marmelade on the underside of a macaron and cover with another macaron. Repeat with the remaining macaroons and marmalade.
The macarons will keep in an airtight container for about 1 month without marmalade or 2 weeks with marmalade. They also freeze well.