Even for objects composed of the same materials, variances may be seen in the colors due to differences in the gloss of the surfaces.
For example, why is a duller blue color seen when sandpaper is applied to a shiny or high-gloss blue sample?

When a ball bounces on a wall and returns, it bounces and returns at the same angle. In the same manner, the light, which reflects at the equal but opposite angle from the light source is called the specularly reflected light. This specular component is reflected as if reflected by a mirror. The light that is not specularly reflected, but scattered in many directions, is called diffuse reflectance. The sum of the specular reflectance plus the diffuse reflectance is called the total reflectance.

For objects with shiny surfaces, the specularly reflected light is relatively strong and the diffused light is weaker. On rough surfaces with a low gloss, the specular component is weak and the diffused light is stronger. When a person views a blue plastic object with a shiny surface at the specular angle, the object does not appear to be as blue. This is because the mirror-like reflectance from the light source is added to the color of the sample. Usually, a person looks at the color of the object and ignores the specular reflection of the light source. To measure the color of a specimen in the same manner that it is viewed, the specular reflectance must be excluded and only the diffuse reflectance must be measured. The color of an object can appear different because of differences in the level of specular reflectance.

It was understood that the color is viewed differently if the surface condition of the object is changed because people only view the diffused light. However, the colors of objects should not be changed because the materials themselves are the same. How can we recognize the color of the materials themselves?
The amount of specular reflectance and diffuse reflectance changes depending on the surface of the object. However, the total amount of reflected light is always the same if the materials and color are the same. Therefore, if a glossy blue plastic part is sanded, the specular reflectance is reduced and the diffuse reflectance increases. This is why the total reflectance (specular plus diffuse) should be measured.

The position of the light trap in Conditions 3 (SCE) and 4 (SCE), as displayed in Figure 24 in Part III-1 , show how the specular reflectance is excluded from the color measurement of the sample. If this trap is replaced with a white plug, as in Conditions 5 (SCI) and 6 (SCI), the specular reflectance will be included in the color measurement. The method of color measurement, which excludes the specular reflectance, is called SCE (Specular Component Excluded). If the specular reflectance is included in the color measurement, by completing the sphere with a specular plug, it is called SCI (Specular Component Included).
In SCE mode, the specular reflectance is excluded from the measurement and only the diffuse reflectance is measured. This produces a color evaluation which correlates to the way the observer sees the color of an object.
When using the SCI mode, the specular reflectance is included with the diffuse reflectance during the measurement process. This type of color evaluation measures total appearance independent of surface conditions.
These criteria must be thoroughly considered when an instrument is selected. Some instruments can measure both SCE and SCI simultaneously.