It is vital to learn the science behind histology to understand it. Several different dyes are used in the study of tissue, and each has distinguishing features that determine how they can be utilized. Table 1 outlines a classification system based on these features.
Table 1. Classification of Biological Stains.| Methods by Which Dyes are Classified | Distinguishing Factor |
| Origin | - Natural
- Synthetic (can also be referred to as artificial)
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| Chemical structure | - Molecular structure correlates to the color of the dye
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| Physical or chemical properties | - Basic: has a pH above 7 and has an affinity for acidic tissue components (acidophilic)
- Acidic: has a pH less than 7 and has an affinity for basic tissue components (basophilic)
- Neutral: has a pH equal to 7
- Metachromatic: multiple colors with one dye
- Fluorescent: can fluoresce when exposed to ultraviolet light
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| Staining application | - Vital: staining living tissue
- Postmortem: staining tissue that has been fixed
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| Mechanism of action | - Absorption: dependent on both the charge of the dye and the tissue
- Direct staining: dye differentially absorbed into the tissue
- Indirect staining: tissue is over-stained and then a process called differentiation is used to remove the excess stain
- Mordant: employs a metal to attach the dye to a tissue strongly
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