Coat an 8x4x2-inch or 9x5x3-inch loaf pan with nonstick cooking spray; set aside. In a small bowl, sprinkle gelatin over water; let stand 5 minutes (do not stir).
In the top of a double boiler, whisk together 1/2 cup sugar and egg whites. Add gelatin mixture. Cook over simmering water for 4 to 5 minutes or until mixture registers 160 degrees F on an instant-read thermometer, whisking constantly. Transfer to an 8-cup glass measure. Cover and chill about 1-1/2 hours or until mixture is partially set (consistency of unbeaten egg whites).
Add almond extract to egg white mixture; beat with an electric mixer on high speed about 10 minutes or until mixture is very light and fluffy and it has increased to 5 cups in volume. Remove 1-1/2 cups of the mixture; tint it pink with red food coloring. Spoon into bottom of prepared pan, spreading evenly. Chill for 10 minutes. Divide remaining egg white mixture in half. Stir chopped almonds into one half. Spoon over pink layer in pan. Chill for 10 minutes. Tint remaining half of egg white mixture green with green food coloring. Spoon on top of white layer in pan. Cover and chill at least 3 hours or overnight.
Meanwhile, for custard sauce, in a small saucepan, combine egg yolks, 2 tablespoons sugar, and salt. Add milk. Cook and stir over medium heat until mixture thickens and coats the back of a clean metal spoon. Set saucepan in a large bowl partially filled with ice water. Stir custard for 2 minutes to cool quickly. Stir in vanilla. Cover surface of custard sauce with plastic wrap; chill for up to 24 hours or until ready to serve.
To serve, invert pudding onto a serving platter; cut into slices. Serve slices topped with custard sauce. If desired, garnish with toasted almonds.