Preheat oven to 300˚F. Grease 8x8 pan and line with parchment paper.
Cream butter until soft (1-2 minutes). Add food coloring if desired. Add pudding mix and sugar and cream together until well combined. Add flour and mix until a dough forms. Add ½ cup pistachios and mix until just combined.
Press mixture into pan and flatten with an offset spatula (or a measuring cup). Sprinkle with remaining pistachios and press lightly into dough. Score dough into desired number of pieces (16 squares, or 9 squares, and each square in half diagonally).
Bake 30-35 minutes. Cool 5 minutes, then cut through (while warm). Allow to cool and remove from pan.