Cyanotypes is a process that uses potassium ferricyanide and a UV-sensitive iron(III) salt (e.g., ferric ammonium citrate).
 Cyanotypes work because the UV-sensitive iron(III) salt, when exposed to UV light, is reduced to iron(II). In water, this iron(II) then reacts with potassium ferricyanide to form Prussian Blue (Fe₄[Fe(CN)₆]₃), which develops the image. Optionally, the print can be developed further by soaking it in a low-concentration hydrogen peroxide solution to oxidize the Prussian Blue and produce a more vivid color. You can also bleach the print (using something like sodium carbonate to remove color) and then treat it with tannic or gallic acid to create different hues.
 Cyanotypes don’t release much free cyanide, so they’re not very dangerous. Potassium ferricyanide itself isn’t highly toxic; it’s even used as an anti-caking agent in salt. However, when Prussian Blue or potassium ferricyanide come into contact with strong acids like concentrated hydrochloric or sulfuric acid, the iron and cyanide separate, releasing hydrogen cyanide gas and cyanide salts, which are extremely hazardous.

Citritypes, on the other hand, are based on the chemical S.F.C (sodium ferric citrate).
 Citritypes work like cyanotypes: S.F.C is an iron(III) compound, and UV light reacts with it — S.F.C is reduced to iron(II) ions, then a developing wash is used. The developing wash is made by submerging the citritype in water with sodium carbonate and an antioxidant (the best one is sodium thiosulfate, as it doesn’t create complexes with iron(II) or iron(III)). The S.F.C is washed away so it doesn’t react with UV anymore. The sodium carbonate makes a complex with the iron(II) ions, forming iron(II) carbonate, which gives the image a greenish-grayish color. The antioxidant helps prevent it from looking muddy.
 It’s not required, but there are two ways (that can even be used at the same time) to help the colors not fade:

⚠️ Safety Precautions (for Citritypes chemicals)