Preheat oven to 350 degrees. Butter a 13 X 9 inch baking pan and line the bottom with parchment/wax paper, leaving a 2 in ch overhang on the ends (easier to remove), butter the paper.
Beat egg whites until they hold stiff peaks. Add 1/4 cup sugar (a little at a time) beating at high speed until whites hold stiff, slightly glossy peaks. Set aside.
In another bowl, beat together almond paste and remaining 3/4 cup sugar until well blended, approximately 3 minutes. Add butter and beat until pale and fluffy, another 3 minutes. Add yokes and almond extract and beat until combined well (approx. 2 minutes). Reduce speed to low, then add flour and salt and mix until just combined. Fold egg whites into almond mixture to lighten the mixture.
Divide batter evenly among 3 bowls. Stir red food coloring in one bowl; green in the other; leaving one bowl uncolored.
Pour one bowl/batter into creased baking pan and spread evenly with offset spatula (layer will be about 1/4 inch thick). Bake 8 to 10 minutes until the edges just pick up color - do not overbake. REPEAT WITH REMAINING bowl/batter - Transfer layers to a wire rack to cool.
When all layers are cool, invert green onto a parchment or wax-paper-lined large baking sheet. Discard paper from layer and spread with half of preserves. Invert white on top of green layer, discarding paper. Spread with remaining preserves. Invert red layer on top of white layer and discard wax or parchment paper.
Cover with plastic wrap and chill several hours (at least 4 hours). (Recipe calls for you to weight the top down with a heavy object but I stopped doing that and it comes out fine.) Remove weight and plastic wrap. Bring layers to room temperature. Melt chocolate in a double boiler or a metal bowl set over a saucepan of barely simmering water, stirring until smooth. Remove from heat. Keep chocolate over water.
Trim edges of cake and pour/spread chocolate over the top in a thin layer. Chill, uncovered, until chocolate is firm. OPTIONAL - you can flip the cake over and coat the underside with chocolate as well. Chill again.
When all is chilled, cut lengthwise into strips, then crosswise. TIP: THE COLDER THESE ARE, THE EASIER TO CUT ... MAKE SURE THEY ARE WELL CHILLED / EVEN SLIGHTLY FROZEN!