The feathers of many birds are blue, not because they produce a pigment, but because the microscopic structure of their feathers is able to filter light. This physical phenomenon is interesting ...

The dye has a structure similar to the known blocker, Nedergaard says. And the dye is very closely related to FD&C Blue No.1, which is considered very safe, with no toxicity in doses up to 12 ...

And 6,6'-dibromoindigo, which is the "official" name for the dye is dark red, not purple. Huh? Simple answer? Purple + light red + dark red + blue = Tyrian purple. Structure-Color Relationships.

Once the only blue dye in Europe, woad pigment made a fortune for the sun-rich Toulouse area during the Renaissance, when it also was called Toulouse’s Blue Gold.