Two purchase orders can both read "Pigment Red 122, 25 kg" and still arrive as different products. The chromophore — 2,9-dimethylquinacridone, C22H16N2O2, CAS 980-26-7 — is fixed by the name. Almost everything that decides whether it performs in your ink, coating or polymer is not. The Colour Index is a naming system, not a specification, and the gap between the two causes most colour complaints.

A name identifies the molecule; a grade identifies the product

Every CI generic name has a companion CI Constitution Number, and both belong on the enquiry. The generic name groups substances by colour and application class; the number ties them to a structure. Neither says anything about crystal phase, particle size, surface treatment or the dispersant sitting on the powder. Suppliers carry those as grade codes, and two grades of one CI name can differ enough that one flocculates where the other stays open. When a catalogue lists the same name twice — Pigment Red 122 appears twice on this supplier's site, and so does Indigo — it is usually not a clerical error but two grades or two physical forms. Only the datasheets separate them, as the second Red 122 entry shows.

Pigment or dye is decided by the medium, not the molecule

A dye dissolves in the vehicle it colours; a pigment does not. Copper(II) phthalocyanine, C32H16CuN8, CAS 147-14-8, is the clearest example here. As the insoluble blue it is Pigment Blue 15, the workhorse of inks and coatings and, in its beta crystal form, a green-shade blue with high hiding power that alkyd systems still specify. Sulphonate the same ring system and it becomes a water-soluble dye. The same test puts Acid Blue 9, C37H39N3O9S3, CAS 2650-18-2, on the dye side: supplied as the ammonium salt, it is the Brilliant Blue FCF approved for food as the sodium salt. Crystal Violet and Congo Red are likewise soluble, which is why they are handled as dyeing and indicator chemistry.

Where the suffix number comes from

Precipitate a soluble dye onto an insoluble substrate — aluminium hydroxide, barium sulphate, calcium carbonate — and you have a lake, a pigment that behaves like one. That is why colour names sometimes carry a colon and a digit: the number records the substrate and laking route. Pigment Red 81 is a rhodamine lake, and its formula is written with an open-ended counter-ion for that reason. Pigment Yellow 191 carries calcium inside the formula, C17H17CaClN4O7S2: the metal is part of what you are buying.

Crystal phase decides the performance

One molecule can crystallise into phases with different colours and different fastness, and the phase is a process variable the supplier controls. The quinacridones are the standard illustration: the commercial Pigment Red 122 phase gives the bright blue-red that displaced cadmium reds in high-value plastics, and its selling points are migration resistance and heat stability rather than shade alone. The phthalocyanines repeat the pattern, with Pigment Green 7, C32Cl16CuN8, at the fully chlorinated end of the family where the shade is locked to green.

Carbon black is bought by structure, not by colour

Every carbon black shares one CI name and one element symbol, which makes the name useless as a purchase specification. Carbon Black, CAS 1333-86-4, is bought on four numbers instead: primary particle size in nanometres, structure as DBP or OAN absorption, surface area by BET, and volatile content or pH for the system at hand. Low structure gives jetness, gloss and easy dispersion; high structure raises viscosity and conductivity. Tint strength is measured against a retained reference, and sieve residue predicts screen and filter blockage better than any colour card.

The inorganic workhorses and the effect range

Beyond organic chemistry sit pigments whose limits are chemical rather than optical. Pigment Blue 29, the ultramarine, is a sodium aluminium sulphosilicate: outstanding light and heat fastness, and no tolerance for acid, which decomposes it with hydrogen sulphide, or for strongly alkaline lime systems. Prussian Blue is the mirror image — a ferric ferrocyanide that resists acid and is bleached by alkali. Mica is bought as a substrate, not a colourant: the muscovite platelet that carries the oxide coating in pearlescent pigments. Strontium aluminate, SrAl2O4, is the long-persistence phosphor that displaced zinc sulphide in glow-in-the-dark work because the afterglow lasts for hours rather than minutes.

Vat colours are not powders in the same sense

Indigo, C16H10N2O2, is insoluble as sold. It reaches the fibre only after reduction to the soluble leuco form in alkaline dithionite and oxidation back to pigment inside the fibre — the vatting step behind denim's surface ring. Vat Blue 4, Vat Green 8 and Vat Yellow 33 follow the same chemistry and give the highest wash and light fastness available on cellulosics. Where colour is needed in a plastic melt rather than on a fibre, Solvent Blue 36 dissolves in the polymer and gives transparent shades no pigment can.

Food and cosmetic colourants answer to a different rulebook

For anything ingested or applied to skin, the question is not fastness but whether the colorant is on the permitted list for that market, at what maximum level. Carminic acid, C22H20O13, is the cochineal anthraquinone behind carmine and E120; gardenia yellow, built on the crocin skeleton, is the workhorse of flour products and confectionery in East Asia; pelargonidin chloride is an anthocyanidin standard, soluble in ethanol and turning red in dilute hydrochloric acid, and it shares the anthocyanins' sensitivity to heat and pH. None of these is interchangeable with a technical pigment.

Fastness is a set of separate tests

"Good lightfastness" is not a specification. Ask for the Blue Wool rating at full shade and at a reduced tint, because the tint figure is always the weaker of the two. Add heat stability as minutes at a stated temperature in a named polymer, migration resistance in plasticised PVC and against a white lacquer, plus acid, alkali and solvent resistance. Then verify the grade by name: Pigment Yellow 147 and the cheaper Pigment Yellow 83 are not alternatives in a high-heat engineering resin, even though both match on a drawdown at room temperature. Where the colour comes from a diazo route the intermediates matter too — Pigment Orange 13 starts from dichlorobenzidine, and 1-methylamino-4-bromoanthraquinone is a typical intermediate.

What goes on the enquiry

Eight lines make quotations comparable: the CI generic name with its constitution number, and the CAS; the physical form, whether powder, filter cake, dispersion or masterbatch; fastness targets written as numbers for your medium rather than as adjectives; a crystal phase or a grade reference to a sample you have approved; particle size, or fineness, with the sieve residue; oil absorption or DBP where relevant; compliance status for your market, tested to the standard that applies; and a batch colour tolerance as a maximum colour difference under a stated illuminant and instrument, against a retained reference. The Colour Index tells you which colour family you are buying; everything else has to be written down.