Colourants listed under a C.I. Generic Name whose serial number contains a colon differ slightly from those listed under the parent C.I. Generic Name. This slight difference is usually chemical but may be due to different crystal modifications in the case of certain pigments.
Historically, a colon number was added to the serial number of a particular C.I. Generic Name when a new product was reported by a manufacturer as being similar to the original. ‘Similar to’ can generally be interpreted as meaning slightly different chemically. This imprecise definition gave manufacturers the opportunity to request new C.I. Generic Names for products that should have been given colon numbers. Conversely, subsequent disclosures revealed that certain products that had been given colon numbers should really have been allocated new generic names. The system has, nevertheless, been continued where it was considered helpful to users to group certain colorants together in this way.