All of a sudden, I think this is very interesting. However, the price of green ferric ammonium citrate is very expensive. One of the unsubstantiated ratios mentioned on Wikipedia is 8.1% (w/V) potassium ferricyanide aqueous solution and 20% ferric ammonium citrate. 6 drops of 1% (w/V) potassium dichromate solution can be added for every 2ml as sensitizer. See Wikipedia for details

Phenolphthalein:

Cyanotype is usually the first classical/non silver photography process that people encounter, because it is very simple and can produce successful photos in a short time. The basic blue sun method requires two kinds of stock containing a certain percentage of solution: 20% ferric ammonium citrate solution and 10% potassium ferricyanide solution (green citrate crystal is more sensitive than brown crystal). Mix the same amount of solution and spread it on the paper evenly. Then dry the paper in the dark. Put the photo paper under the negative with strong contrast for exposure. The image appears about 15 minutes in direct sunlight. This is the first picture of my successful experiment: 22 and 33 mothers