Green colours are located in the region of 490-560nm and yellow colours fall between 560-590nm.
During daylight conditions, human visual sensitivity peaks at 555nm, then shifting to 510nm when light is low. The reflectance values for lime-yellow paints (such as DuPont Imron 7741™) peak at 550nm during daylight and between 520-530nm at night. These reflectance values almost exactly match the peak human sensitivity curve under all viewing conditions (both photopic and scotopic vision).
The eye resolves green-yellow better than any other colour. Green-yellow is unaffected by the Krovkov effect. The colour performs well in fog and under adverse weather conditions such as heavy rain and overcast skies. Green-yellow is virtually unknown as a colour in nature and is rare as a motor vehicle colour. It contrasts with almost every urban and rural background.
Solomon states (p71) that a comparison between DuPont Imron lime-yellow (7744)TM and Imron red (674)TM paints demonstrates that lime-yellow has a peak eye response approximately 4.9 times greater than red during daylight conditions. At night, the response is approximately 93 times greater, due to red being practicably invisible at night.
In a lateral angle situation when two vehicles are crossing an intersection at 90 degrees to each other, lime-yellow is detected significantly earlier than red, providing a longer time period to react and initiate an avoidance response.
The second-best body colour after yellow-green is pure yellow at 560-590nm.
This wavelength is just above the peak daylight sensitivity on the spectral curve. Green (p7) states that yellow possesses an intense chromacity, plus high luminosity, performing almost as well as green-yellow in fog and bad weather. Yellow does occur frequently in nature, but is usually found only in small amounts. A yellow vehicle presents with a much larger area of colour and contrasts with most backgrounds.
Chrome or golden yellow is the usual paint description. This colour is also commercially available for use on public vehicles. In the lateral angle situation mentioned above, yellow is perceived in the interval between red and lime-yellow. People suffering from colour blindness may have trouble seeing red and green, but they easily detect yellows. Yellow is also often used for maritime visibility.
The red wavelengths are between 630-760nm.
Red is a highly emotional colour located at the margin between visible light and infra-red frequencies. The sensitivity of human photoreceptors to red is small and a high level of red input is needed to penetrate beyond the level of the receptor’s threshold. Red wavelengths are almost imperceptible when night adaptation takes place after the Purkinje shift (scotopic vision). Red = black at night!
Traditionally, red is the emergency service colour, particularly for fire brigades. Often red has been combined with white in a two colour design. The belief was that the superior reflectivity of white would increase vehicle visibility and offset the poor performance of the red colour panels, especially in low light. In fact, the two colours did the opposite. They combine to create a double camouflage by disrupting the vehicle’s silhouette. The red is difficult to perceive and the white merges with the surrounding background. This combination distorts perception and slows the sensory process, thus increasing reaction times. Chromatic aberration causes the divided two-colour segments of the vehicle body to be perceived differently by the eye as the incoming wavelengths of light (colour) find a different focus point at varying distances from the fovea.
The red and white colour combination has been placed 10th out of 12 different colours paired in visibility research conducted by the USA National Safety Council (Solomon, p68).
Orange is similar in response to red, but to a lesser degree.
Orange-red colours are often used for highlighting vehicles against snow and the enhanced visibility of vessels or liferafts against a dark sea. American KKK specifications specify a mid-orange colour (Omaha Orange) for the waistline stripe on national ambulances. The orange KKK feature has no value or negative effect on increasing vehicle visibility.
The now common but unusual fluorescent (or Day-Glo®) colours are centred on the orange-red portion of the spectrum, providing enhanced conspicuity over popular paints with normal colour pigments. These ultra-violet (UV) sensitive pigments increase in brilliance at twilight, due to the higher levels of UV radiation from the sun at this time of day.
Green falls between 490-560nm.
The marginal yellow-greens possess high reflectivity, their effectiveness already explained. Greens from the lower blue-green regions become the safety or “go” colour and then darken further. Greens in the mid to lower wavelengths are everywhere in nature. A vehicle with some mid-green or dark-green panels will suffer reduced contrast against many country and urban backgrounds; the silhouette of this vehicle will often be broken up by camouflage effects. However, dark green typefaces written as text on white backgrounds produce a highly visible text message, second only to black or blue colours for legibility.
Blue is generally considered too dark to use as an overall body colour. However blue has a lower threshold under rod vision and a higher spectral sensitivity than red or orange under all conditions. Remember that under rod vision there is no colour, just brightness. This translates to blues being seen before the traditional fire engine red colour and this casts doubts on the popularity of red. The reflectivity values for blue are enhanced by the Purkinje shift towards the lower wavelengths. Blue is the traditional police and authority colour in Australia and many other countries. Blue body panels on vehicles appear as grey-black in low light.
White is not a colour, but is composed all visible spectral wavelengths reflected together.
It is considered the third choice after yellow-green and chrome yellow for vehicle body colour. Emergency services have used white alone or in combination with other different colours for many years. White has extremely high reflectance and achromaticity values. Although, this achromaticity also allows white to blend with backgrounds such as overcast sky, fog, rain, snow (Green, p7). Jack Murray (p3) agrees that white… is very dangerous in foggy weather. White is passive and a neutral white may alter its own “colour” values under some artificial light sources (see below). White usually requires some enhancement, eg. stripes, to increase visibility under the weather conditions mentioned above.
White is always a popular and common colour-of-choice with many car owners. While it is reasonably high in visibility and the shape notwithstanding, a white emergency vehicle in traffic may be surrounded by dozens of similar white vehicles sharing the same section of road. Thus the context of identical surrounding colour will always reduce the visual impact. Fortunately white can be combined with the yellow-greens to increase conspicuity. This is because the reflectivity values of the two colours are similar and the level of chromatic aberration between these two colours is very low. Often fitted, a common solution is to use yellow-green striping on the vehicle to increase visibility. This maintains white as the major body colour with markings covering less than 10% of total vehicle surface area.