It doesn't give details on the what kind of coloring was used, I assume just the standard grocery store food coloring available in the mid 1960's. All the colors utilized the same batch of material so it seems it was just self suggestion influencing people's experiences.

It just goes to show that the material is only part of the story and our expectations and beliefs have more influence than we give them credit for.

A lot of the old chemists believed that your intentions were crystallized into the structure of the sacrament and would come through in the experience.  Lore has it that family chemists still pray over each and every batch.