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A4 are said to be of a similar hue, as are the B, C, and D shades. The recommended technique is to choose the nearest hue first and then select the appropriate match of chroma and value from the tabs available.
It may be difficult to determine a given hue accurately if its chroma or intensity is low. Therefore the region with the highest chroma (i.e., the cervical region of canines) should be used for initial hue selection.
Chroma selection
Once the hue selection has been accomplished, a choice is made of the best match of chroma. For example, if a B hue is determined to match best in color variety, there are four available gradations (tabs) of that hue: BI, B2, B3, and B4.
Several comparisons are usually necessary in determining which sample best represents the blue and its corresponding saturation level. Between comparisons, glancing at a blue object will rest the operator' s eye and help avoid fatigue of the retinal cones.
Value selection
Finally, a determination is made for value with a second commercial guide whose samples are arranged in order of increasing lightness. By holding the second shade guide close to the patient, it should be possible to determine readily whether the value of the tooth is in the range of the shade guide. Attention is then focused on the range of the shade that best represents the value of the tooth and how that range relates to the tab matching for hue and saturation. Observation from a distance, standing slightly away from the chair, and looking through squinted eyes will help in assessing the value. By squinting, the observer can reduce the amount of light that reaches the retina. Stimulation of the cones is reduced, and a greater sensitivity to achromatic conditions may result. While squinting, the observer concentrates on which disappear from sight first, the tooth or the shade tab; the one that fades first has the lower value.
When the proper selection of value has been made, it will be the exception rather than the rule for this to coincide with the determinations for hue and chroma. A decision must be made whether it is necessary to change the preciously selected shade sample. If the independent value determination is lower than the value of the sample selected for hue and chroma, a change is usually necessary since it is not possible to increase the value of and object by adding surface stain (which always reduces brightness). If the value determination is higher than the hue determination, it should be decided whether this difference can be bridged through internal or surface staining of the restoration. The determinations of hue and chroma as well as of value are them communicated to the laboratory.
Shade distribution chart
Shade distribution charting is a practical approach to accurate shade selection and is recommended even when a fairly good match is obtainable from the commercial shade sample.
The tooth is divided into three regions: cervical, middle, and incisal. Each region is matched independently, either to the corresponding area of a commercial shade sample or to a single color porcelain chip. Because only a single color is matched, intermediate shades can usually be estimated rather easily and duplicated by mixing porcelain powders. The junctions between these areas are normally fairly distinct and can be communicated to the laboratory in the form of a diagram. The shade distributi