FC and FD reagents are both almost the same because they contain phosphotungsatae and phosphomolybdate, but FC is more optimized to give more intense blue/green color when reacting with phenolics.
FC has lithium sulfate to reduce the white salts precipitate that form upon reacting with FD, and make the FC solution more intense yellowish one.
Molybdate ratio to tungstate is higher in FC than FD in order to achieve the above mentioned purpose.
the wave length used in FC (730nm) with not interfere with pigmented plant sample matrix.
The Folin-Denis reagent is prepared by mixing sodium tungstate and phosphomolybdic acid in phosphoric acid. Folin-Ciocalteu reagent (aka Folin's phenol reagent) is just a modification of the Folin-Denis reagent. The modification consisted of the addition of lithium sulfate and bromine to the phosphotungstic-phosphomolybdic reagent.
They are both used for the colorimetric in vitro assay of polyphenolic antioxidants.