These instructions are for traditionally-colored Italian rainbow cookies. The photos are of my green ombré type. You can choose whatever colors you like for the layers; see the Recipe Notes at the bottom.
Line three 9×13 cake or sheet pans with wax paper or parchment paper and grease with butter or spray with cooking spray.
Break up almond paste in a large bowl, add butter, egg yolks, sugar and vanilla. Beat until light and fluffy. Beat in flour and salt. Set aside.
In separate bowl, beat egg whites to stiff peaks. Fold into almond mixture.
Separate batter into equal parts (about 1 1/2 cups each) in three different bowls. Add green color to one bowl and mix well. Add red color to another bowl and mix well. Leave third bowl plain.
Spread batter into separate pans.
Bake each pan for 8 minutes then turn and bake 7 minutes more, for a total of 15 minutes. Place pans on racks until completely cool.
Invert green layer onto a large tray. Spread with half the raspberry preserves.
Invert the plain layer on top and spread with other half of preserves. Invert red layer on top.
Wrap in plastic wrap, place a heavy book or pan on top and refrigerate overnight.
Combine chocolate chips, 4 tablespoons butter and light corn syrup over low heat and stir until melted.
Cover the top of the cake with chocolate glaze and refrigerate until glaze sets. Turn over and cover the other side with glaze and let cool until that side sets.
With serrated knife, trim edges of layers so they are squared.
Cut the cake crosswise into eight sections about 1 1/2 inches wide. Then cut it lengthwise into twelve 3/4-inch sections to make about 96 small, layered cookies.
Make the cookies ombré by adding 5 drops of any color to one third of the batter, 10 drops of the same color to another third of the batter, and 15 drops to the final third. Eyeball it to see if you like the color gradation; remember that it's easy to add color, impossible to subtract it. Combine with darkest layer on plate, lighter layer in the middle and plain layer on top. Use seedless jam or preserves that match the color you choose or use a neutral-colored jam like apricot so the lines aren't distinct.