True color in chemistry is the color seen in a solution or compound in standardized conditions. It depends on electronic transitions in molecules, specifically absorption of a particular wavelength of light, and this depends on the structure of the compound.

The automotive, aerospace, metal finishing, marine, construction, and manufacturing industries utilize and rely on coating thickness gauges the most. They ensure that coatings are within the expected criteria for functionality, safety, and durability.

The machine used to measure color is primarily called a colorimeter or spectrophotometer. 

Spectrophotometer: The most common and precise type. It analyzes light reflected/transmitted by an object across the visible spectrum to quantify color accurately. A spectrophotometer can measure colors on smooth or matte surfaces, as well as textured, glossy, mirror-like surfaces, and special effect colors. It measures the reflected light of a sample at a fixed angle (e.g., 45˚) or captures light reflected at all angles to calculate color measurements that closely match what the human eye perceives. Additionally, similar to how humans flip a sample to view colors from different angles, a spectrophotometer is suitable for measuring a variety of materials and surface characteristics. Widely used in industries like paint, textiles, plastics, Chemicals, Pharmaceuticals, and printing. 

Colorimeter: Also called photoelectric integrating colorimeter, a simpler, more cost-effective option. It measures color based on three primary colors (RGB) and is suitable for basic color matching needs. A photoelectric integrating colorimeter is a color measurement device based on the photoelectric integration principle. It directly measures the tristimulus values XYZ of an object's color using three color filters (red, green, blue) and silicon photocells as three sensors. The color measurement principle of this instrument imitates the human eye's mechanism of perceiving the three primary colors (red, green, blue). It corrects the relative spectral sensitivity of the detector through color filters to match the CIE-recommended spectral tristimulus value functions x(λ), y(λ), and z(λ).

Spectrocolorimeter: Combines the functions of spectrophotometers and colorimeters, offering both spectral data and color space values for comprehensive analysis.

 A spectrophotometer illuminates a sample with light or passes light through it and records the reflected or transmitted light over the wavelength range. It transforms this data into color coordinates (such as L*a*b*), allowing accurate color assessment as well as color comparison to standards.

Haze captures the scattering of light, transmittance measures the light that passes through a said material.

The principle of colorimetry is the law of Beer-Lambert, which says that the intensity of light absorbed by a colored solution is proportional to the concentration of the absorbing species and the path length. It measures the extent of light that is absorbed at certain wavelengths.

To quantify color change, take the original L*a*b* values of a sample, and reread after exposure or processing. Compute the difference as 1/2(Emut1 Emut2). The larger the value of ΔE, the more obvious the change of color is, which can be used in quality or stability testing.

Keep light booths clean, maintain the light source lamps, readjust the calibrated light source, and ensure that dust and fingerprints do not settle on the viewing area.

Gloss level is not given out in percentage but in gloss units (GU). In practice, however, 100 GU is considered 100 percent reflective. To contrast visually, the 20-40 GU is a low-gloss surface, and 85 or more is almost 100 percent mirror-like reflection.

To ensure a consistent color perception, observers should look at samples at a 45° light angle and 0° viewing angle to reduce glare.