Figure 6
From: Identification of pigments by multispectral imaging; a flowchart method
Prussian blue is IR absorbent. In the infrared image phthalo blue appears bright because it is IR reflective. On the other hand, Indigo appears bright because it is IR transparent. Indeed, when it is overlapped with Prussian blue it looks dark. Consequently, the infrared false color of the indigo swatch looks not the ordinary bright red but as dark as Prussian blue.