I16. Iridescent Calligraphy Ink Dr. Ph. Martin's Iridescent Calligraphy Colors
These are great. These colors flow out of a broad edge pen like ink. 1oz glass bottles with dropper top. Colors can be mixed. Truly waterproof when dry. The manufacturer says they are lightfast. Copperplate Gold is the most popular color (Copperplate Gold is listed here as "Gold").
Use rubbing alcohol to thin for better flow in your pens.