Andy Mclean posted a comment to my story of copper phthalocyanine (Monastral blue). The issue was its colour, and more specifically why this pigment has two peaks λmax 610 and 710nm making it blue. The first was accurately reproduced by calculation on the monomer, but the second was absent with such a model. Andy suggested this latter was due to stacking. Here, the calculated spectrum of a stacked dimer is explored.