Dyes are generally defined along the lines of being coloured, aromatic compounds that can ionise. They are thus able to interact with oppositely charged tissue constituents. Dyes which are positively charged, and are used to bind to negatively charged tissue components, are termed basic dyes.
The Colour Index uses this as a classification and naming system.
Each dye is named according to the pattern:–
These dyes are thereby specifically identified as basic dyes of the stated colour, and whose primary mechanism of staining is by ionic bonding. Note that this is a functional and colour classification. It contains no chemical information, neither does it imply that dyes with similar names but unique numbers are in any way related. It should also be noted that the classification refers to the primary mechanism of staining. Other mechanisms may also be possible.
Basic dyes have amino groups, or alkylamino groups, as their auxochromes, and consequently have an overall positive charge. In other words, the coloured part of the molecule is the cation. Although the molecular charge is often shown on a specific atom in structural formulae, it is the whole molecule that is charged.
There are numerous other examples.