al importance of the warm/cool contrast probably arises in the capability of the human visual system to adjust to changes in the color and intensity of natural illumination under different phases of daylight. In response to these illumination changes, color vision changes both its relative sensitivity to light (chromatic adaptation) and its relative judgment of colors (color constancy) to maintain a consistent appearance in surface colors.
A heightened awareness of the warm/cool contrast in painting technique probably dates from the representation of diurnal and climatic changes of light in late 17th century landscape paintings, and was explicitly extended in artistic practice during the 18th century.
What are these changes in natural light? Natural daylight changes in two ways: brightness and chromaticity (color). Under clear skies, brightness ranges from a high around noon to a low just after sunset. The chromaticity changes, as defined by the correlated color temperature, range from a greenish white at noon to an intense scarlet at sunset (for direct sunlight), or from a cerulean or greenish blue at noon to a deep yellow at sunset (for sunlight and skylight combined).
Light intensity also has specific warm/cool effects as it interacts with different kinds of materials. In general, as the intensity (illuminance) of light on or through materials increases, the apparent color of the materials shifts toward yellow: blues become blue green, greens become yellow green, oranges become yellow, reds become orange; violets shift either toward red or toward blue, depending on the hue balance. The effects of reflected, filtered or shadowed light change in the opposite direction, from yellow to red or green blue to blue. (For example: layers of translucent yellow material redden as they become thicker, greenish pools darken to blue green as waters get deeper, etc.)
What are the warmest (coolest) hues? There is no such thing as a "warmest" or "coolest" hue in colorimetry. Two traditions have developed in representational painting. Emphasizing the chromaticity changes across natural daylight phases suggests a warm/cool contrast anchored on red orange. This makes the warmest hue a paint such as pyrrole scarlet (PR255) or pyrrole orange (PO73); the coolest hue is a greenish blue such as phthalocyanine cyan (PB17) or phthalocyanine turquoise (PB16).
Emphasizing the intensity changes in natural light suggests a warm/cool contrast anchored on deep yellow. This makes the warmest hue a paint such as hansa yellow deep (PY65) or quinacridone gold (PO49); the coolest hue is a (dark) blue violet such as ultramarine blue (PB29) or cobalt blue (PB28).
Are there objective criteria for a warm/cool judgment? Yes. All warm surface colors have three reflectance attributes: (1) a sharp transition or "cliff" in reflectance located between the "cyan" to "yellow" wavelengths; (2) maximum reflectance from this cliff to the "red" end of the spectrum; (3) little or no reflectance from the cliff to the "violet" end of the spectrum. This locates warm colors in the hues from light yellow to deep red.
The "cool" hues are defined in nearly complementary terms: (1) a "cliff" profile located between the "cyan" to "yellow" wavelengths, (2) maximum reflectance from this cliff to the "blue" region of the spectrum, and (3) little or no reflectance from the cliff to the "red" end of the spectrum. The "violet" wavelengths must be excluded from the judgment, as these stimulate the a+ opponent function (appear to contain some "red" light). This locates cool colors in the hues from bluish green to deep blue.
Is every color either warm or cool? No. I feel that greens and purples are in themselves neither warm nor cool, because they do not conform to all three of the three criteria just listed for a warm or a cool color, and because they are not salient in the orange to blue transitions of daylight colors.
However, any color can be relatively warmer or cooler in relation to any other color, depending on which color is closer to the arbitrarily chosen warmest (or coolest) col