Prepare vanilla layer. Combine flour, powdered
sugar, and salt in bowl of food processor, pulse
3 to 4 times till combined. Add butter, pulse
till mixture resembles coarse sand, about 5 times
Add egg yolks and vanilla, process till dough
forms a ball, 30 to 40 seconds. Transfer to
prepared baking pan and press evenly into the
bottom of pan. Brush top of dough with milk.
Prepare pink layer. Prepare as above, reducing
vanilla to 1/2 tsp and adding 2 to 3 drops red
liquid food coloring. Transfer dough to a
lightly floured surface and roll into a roughly 8
inch square. Press pink layer evenly on top of
vanilla layer in pan. Brush top of dough with
milk. Prepare green layer like pink layer, using
green food coloring. Press on top of pink layer.
Cover with plastic wrap and chill 1 hour.
Preheat oven to 325. Line 2 baking sheets with
parchment paper. Lift dough our of pan. Cut
dough into quarters. Cut each quarter crosswise
into 18 slices. Arrange 1 inch apart on prepared
pans. Bake until firm to the touch, 14 to 15
minutes. Do not let cookies brown. Cool on
sheets 5 minutes. Transfer to wire racks to cool
completely, about 20 minutes.