1. Introduction
Color is crucial to the survival and reproductive success of many flowering plants because it dictates whether or not insects will be attracted to and ultimately pollinate the plant [
1]. A flower’s apparent color is governed by the selective absorption of specific wavelengths of light by the petals, as well as by light scattering in the petal’s interior [
1]. Although the specific variation and signaling of different petal colors are not well understood, all petals function in a similar sense, in that their purpose is to reflect sunlight in a way that attracts a pollinator’s attention. However, there is a decreased return in light reflection for increased levels of color, and this fact is compounded by the energy required to produce more pigment [
1]. For this reason, flower petals tend to be translucent rather than an energy-intensive solid, opaque color, as the higher rate of visible light reflection has been shown not to attract more pollinators, although petals that are too translucent would fail to attract pollinators [
1]. Therefore, plants are in a constant act of balancing not attracting enough pollinators with spending too much energy on pigment production, causing variations in a petal’s color value between plants [
1].
With a wide variety of colors and conditions that can affect them, there are many different color standards in use. For this research, the CIELAB color space, also known as CIE L*a*b*, was used. The CIELAB color space is defined by planes of constant lightness, L*, in relation to a net of lines parallel to the a* (green to red) and b* (blue to yellow) axes [
2]. By calculating color on three axes, the CIELAB standard is able to provide an accurate yet uniform color measurement standard [
2]. Unlike the commonly used CMYK and RGB color models, CIELAB is designed to approximate human vision, with the L* value closely matching the human eye’s perception of light [
3]. It also has the advantage of being able to make minute color balance corrections by changing the a* or b* values, and to adjust the lightness contrast through the L* value [
3]. Today, CIELAB is the most complete color space specified by the International Commission on Illumination (CIE) [
3]. The L* value is measured on a scale from 0 to 100, where 0 is black and 100 is white [
3]. The a* value is measured from −127, which is pure green, to 127, which is pure red [
3]. Lastly, the b* value is measured from −127, which is pure blue, to 127, which is pure yellow [
3]. This three-dimensional range between colors allows for distance to be calculated between colors, directly proportional to the difference between two colors in the human eye [
3].
The color of flower petals is important for horticulturalists because, for ornamental plants such as camellias, petal color is one of the most critical characteristics and serves as a target and benchmark for plant breeding [
4]. Petal color is a clear indicator of expressed genes that provides horticulturalists with a valuable pathway towards creating new varieties of ornamental plants, or visually understanding genetic interactions when breeding plants [
4]. Furthermore, when some colors are distinctly lacking from ornamental plants (such as the lack of blue in Chinese roses and chrysanthemums), horticulturalists can profit if they breed new colors in popular ornamental plants, due to the novelty and visual appeal, highlighting the economic value that petal color can garner [
5]. Petal color can also visually indicate pigments and chemicals present within certain species, as major flavonoid pigments such as anthocyanins and other crucial plant chemicals are directly related to a plant’s color [
5]. As a result, petal color serves as a trove of information for horticulturalists regarding a plant’s genetic and chemical makeup, as well as serving as a visual indicator of genetic interactions from breeding. Thus, petal color provides horticulturalists with an invaluable amount of data, as well as the potential for economic success.
A wide array of physical and optical characteristics can dramatically alter a person’s perception of color. Color refers to the part of the electromagnetic spectrum that is visible to the human eye. An object’s color is defined by the visible light that is not absorbed, but instead is reflected into the human eye [
6]. However, color is not absolute, as many physical conditions can alter the human perception of color. Two principal physical conditions are value, the relative lightness or darkness of a color, and gradation, the physical conditions of the contiguous surface [
7]. Both of these conditions are relative, as the value of a color can change with the lighting (i.e., a rainy or sunny day) and the gradient of a color is relative to the surface it is covering. Consequently, human perception of a color can change dramatically just because of a different time of day or due to the presence of a different surface. Furthermore, optical illusions, such as the simultaneous contrast effect, can alter one’s perception of a color due to a surrounding color [
7]. This optical illusion is often seen when a color is placed on a bright background and the color also seems to become brighter in value, despite no change in the actual color.
The Royal Horticultural Society (RHS), the United Kingdom’s leading gardening charity, is one of the oldest and most influential societies in horticulture [
8]. The Society was founded in 1804 for the purpose of publishing scientific research, as well as bringing together the foremost researchers in horticulture. The RHS has expanded its collection of research and experimentation within its scientific gardens, however, its members, as well as gardeners around the world, have struggled with identifying and naming plant colors. Starting in the mid-1930s, the RHS addressed this issue by creating a comprehensive manual containing hundreds of defined colors [
9]. Since then, the RHS Colour Chart has been constantly improved and is now in its sixth edition. In addition to new and refined colors, the chart has been updated from a book to a fan deck, for easier use in the field. Since its introduction, the RHS Colour Chart has been the universal standard for plant color classification.
The RHS Colour Chart works by arranging a spectral order of fully saturated and progressively less saturated colors, which are to be matched to the plant [
10]. The chart functions only if it is used under a natural north light and cannot work with artificial light or direct sunlight. The plant is then placed within the holes of the color chart until a uniform color is formed, which indicates that a matching color and identification has been found. The color chart guide notes that measurements may become increasingly erroneous if the measurer’s eyes become fatigued. Furthermore, flowers, which are not homogeneous in color, will not have an exact match and must be described as being close to the matched color. Once a proper match is found, the type of color can be universally described using the RHS Colour Chart naming system. If an exact match cannot be found, then the natural color of the petal cannot accurately be described [
4]. This is problematic, as often there is not an exact match between a petal and the color chart, but rather the closest equivalent to the petal’s color which is used.
Traditionally, color card matching methods like the RHS Colour Chart has been used as a standard reference for flower color classification. However, these traditional methods rely on human vision to assess color, which varies from person to person. Inexpensive color sensors, such as the Nix
TM Pro sensor (Nix Sensor Ltd., Hamilton, Ontario, Canada), used in the present study, can potentially serve as a more reliable and accurate method of assessing color. The Nix
TM Pro color sensor was developed to replace printed color charts used in interior design and has revolutionized the market by providing an inexpensive yet accurate color sensor within a mobile body [
11]. The Nix
TM Pro color sensor has been used in a wide variety of applications, from soil science [
12] to meat quality checks [
13]. The Nix
TM Pro color sensor was used to identify soil color and predict soil organic carbon and total nitrogen [
12,
14] and soil samples have been compared to the Munsell Soil Color Chart (MSCC) [
15]. The purpose of that study was to evaluate if the inexpensive Nix
TM Pro color sensor could take accurate, consistent readings, potentially resolving issues with the MSCC’s print quality variations and propensity to fade [
15]. The study found that the Nix
TM Pro color sensor provided repeatable readings that were similar to the much more expensive Konica Minolta CR-400 laboratory colorimeter (Konica Minolta Sensing Americas, Inc., Ramsey, NJ) [
15]. The study concluded that the Nix
TM Pro color sensor was an excellent alternative to the MSCC method for in-field soil color determination. However, no previous research could be found that utilized the sensor for plant color evaluation.
A wide variety of species fit into the
Camellia genus. It is home to more than 400 species, with a combination of white, pink, red, and yellow colors. Camellias were originally native to China and Japan’s warm subtropical regions, but were introduced to Western gardens during the colonial period [
16]. Camellia plants have a large economic value for multiple reasons. The primary reason is that camellia plants are harvested for their oil and tea leaves, with camellia oil fetching a significantly higher market price than other plant-based oils because of its nutritional and medicinal properties [
17]. This does not include the large camellia gardening market, which is reliant on accurate color depiction to sell the aesthetically pleasing shrub. Camellias provide a sizeable economic value for their oil and tea yield, as well as through their sought-after beauty. This beauty primarily results from the plant’s flowers, and research has found that petal color is the deciding factor in a majority of consumers’ decisions when buying an ornamental plant [
4]. However, the camellia industry still lacks a widely available and accurate color testing method. Due to the fact that camellias span a vast geographical area, varying environmental conditions can skew human-based color measuring methods. The Nix
TM Pro color sensor could help to alleviate this problem by providing an inexpensive way to identify camellia petal color universally. Due to its patented design, which blocks out light and other environmental conditions, the Nix
TM Pro color sensor could provide uniform readings. This study compares the use of the Nix
TM Pro sensor for petal color classification by comparing it to the current universal RHS Colour Chart identification method. The specific objectives of this study were to: (1) determine camellia petal color with the RHS Colour Chart, (2) assess camellia petal color with the Nix
TM Pro color sensor, and (3) compare the measurements of camellia petal color using the Nix
TM Pro color sensor and 3rd party color equivalents of the RHS Colour Chart.