JARS v62n1 - The RHS mini colour chart: A Belated Review

The RHS mini colour chart: A Belated Review
Donald H. Voss
Vienna Virginia

The RHS mini colour chart (2005) comprises "colours from the RHS Colour Chart specifically chosen for use by the flower trade." The mini includes 244 colors (243 of the 884 colors represented in the 2001 edition of the RHS Colour Chart plus a new White, N999D). The chips in the mini are well matched in color to the corresponding chips in the full set. The mini is a fan of 61 leaves, each with four colors. The grouping of colors on the leaves departs, however, from the arrangement found in the RHS Colour Chart, in which the second, third, and fourth chips on a leaf are generally successively lighter colors of the same hue as the top chip.

Hue Variation on Leaves of the mini
In noting the sometimes large hue variation on individual leaves of the mini , it must be remembered that facilitation of color evaluation with a more limited sample of color space, not hue consistency on each leaf of the chart, was an objective of the compilers. To examine the colors in the mini , measurements expressed in Munsell notation were made using a Minolta CM-2002 spectrophotometer (2° observer, D65, specular excluded). The maximum variation in hue on individual leaves of the chart (excluding the four Black, Grey, and two White leaves) was calculated from the measurements. The hue differences were distributed as follows:

Maximum Difference
in Munsell Hue Notation
Number of Leaves
0.1 - 2.5 18
2.6 - 5.0 21
5.1 - 7.5 14
7.6 - 10.0 1
10.0 = 12.5 3

Differences of 2.5 in Munsell hue (one step in the 40-step hue scale) are readily apparent to persons with normal color vision. A difference of 10 separates the midpoints of Munsell Red-Purple and Red, or Yellow and Green-Yellow, for example.
The large hue differences noted between the four chips on some leaves of the mini raised the question of whether color difference between corresponding chips in the mini and the RHS 2001 chart might be an explanatory factor. To check this, visual comparison of the extreme hues on each of the 18 leaves with Munsell hue differences of 5.1 or greater was undertaken. Eight colors showed slight variation between the two charts, and these were re-measured instrumentally. In six cases, the differences in hue, lightness, or chroma were small, only 0.1 or 0.2 in Munsell notation. Two chips showed larger differences: 69C in the mini measured 3.4RP in Munsell hue (noticeably redder than the 2.9RP for the corresponding RHS 2001 chip); 188A measured 9.9G (noticeably bluer than the 9.1G in RHS 2001). The results confirm that in nearly all instances the sizable hue differences noted on leaves of the mini were a consequence of chip selection rather than error in matching colors of the RHS 2001 chart.
The visual comparison of 18 corresponding chips from the mini and the RHS 2001 chart revealed that surface effect must be considered when using these charts. Several mini /RHS 2001 pairs showed color difference when the viewing angle approached the 0° incident-light angle (perpendicular to the color chip) but not when the viewing angle diverged from the incident-light angle by about 45°. This phenomenon results from differences in the surface condition of the respective chips - smooth or rough, glossy or matte. The chips for which this was noted were 125A, 144A, 139C, and 181A. To avoid error in color evaluation attributable to surface effect, care must be used to view both specimen and color chip at an angle of view about 45° from the angle of incident light.
An additional matter relating to use of the charts is the need for caution with respect to the accompanying instructions. These recommend placing the hole in a color chip over the plant material being described. This procedure facilitates visual evaluation - but at a cost! If the plant material is moist or carries pollen or foreign matter, contaminants may adhere to the back of the chart leaf in use and later be transferred to the next leaf in the fan. This can cause damage that many will prefer to avoid.

Color Names
It has been said that England and the United States are two nations separated by a common language. The truth of this has again been substantiated by the choice of color names appearing on the leaves of the mini . Vernacular color names are notorious for being arbitrary and misleading. For serious work, names based in a scientific sampling of color space are preferable. Such a system of names was created in the United States by the joint effort of the Inter-Society Color Council and the National Bureau of Standards. The ISCC-NBS method of designating colors is based on Munsell notation, which in turn is firmly linked to CIE colorimetric standards. This method offers names that convey to the user a sense of the three elements of color: hue, lightness, and chroma.
In the ISCC-NBS system, pinks (purplish Pink, Pink, and yellowish Pink) are light colors in the hue range that spans reddish Purple - Red - reddish Orange. The meaning of "Pink" in the mini’s color names is problematical; these names are listed below:

Light Red Pink Light Pink
Red Pink Purple Pink
Dark Pink Red Pink Purple
Dark Pink Pink Violet
Pink Dark Pink Violet

The compilers of the mini apply Pink to purplish Red colors, but the ISCC-NBS Pink is a light tint of Red (which the mini assigns to Red Pink). Especially puzzling are Purple Pink and Pink Purple, for which the top chip on each respective leaf is a vivid reddish Purple. A greater divergence relates to the lightness of colors designated in the mini as Pink in its several hue variants. Leaves 12 (Dark Pink Red), 14 (Dark Pink), 15 (Pink), 17 (Purple Pink), and 20 (Pink Violet) include one or two chips that are darker than colors recognized as pinks in the ISCC-NBS system - or, indeed, implicit in a standard U. S. dictionary definition. The chips on leaf 18 (Pink Purple) are reddish Purple and purplish Red, not the lighter purplish Pink colors of the same hue.
Throughout the mini , color-name confusion also results from inclusion of chips of more than one basic hue on a leaf. Thus leaf 1 ("Yellow") has two Yellow and two greenish Yellow chips; leaf 4 ("Orange Yellow"), one Orange Yellow and three Yellow chips; leaf 19 ("Purple"), four purplish Red chips; leaf 23 ("Violet"), four Purple chips; etc., etc.
In the interest of better communication of perceptions of color, the author suggests the use of ISCC-NBS color names in preference to the names at the top of leaves in the mini and in the full RHS Colour Chart. A concordance between the RHS chip numbers and the ISCC-NBS names is available in A Contribution toward Standardization of Color Names by R. D. Huse and K. L. Kelly (ARS, 1984). A list of color-name abbreviations for chips on the 19 leaves added to the RHS chart in 2001 is in the table.

Chip No. RHS 2001
N25A s.O
N25B s.O
N25C s.O
N25D s.OY
N30A v.rO
N30B v.rO
N30C v.rO
N30D v.O
N34A m.R
N34B s.rO
N34C m.R
N34D m.rO
N77A d.gy.P
N77B gy.pR
N77C gy.rP
N77D gy.pR
N79A d.pR
N79B d.pR
N79C d.pR
N79D m.rP
N92A d.gy.P
N92B m.V/d.V
N92C d.V
N92D gy.V/d.P
N134A d.yG
N134B d.yG
N134C s.G
N134D brill.G
N138A m.G
N138B m.yG
N138C m.yG/p.G
N138D m.yG
N144A s.gY
N144B s.YG
N144C s.YG
N144D s.YG
N155A b.Wh
N155B pkWh
N155C pkWh
N155D yWh
N163A deep.O
N163B s.O
N163C s.OY
N163D s.OY
N167A brO
N167B brO
N167C m.O
N167D m.O
N170A brO
N170B m.O
N170C m.yPk
N170D m.yPk
N172A m.rO
N172B m.rO
N172C s.O
N172D m.O
N186A d.pGy
N186B d.gy.P
N186C d.gy.R
N186D d.pR
N187A d.pGy
N187B gy.P
N187C pGy
N187D l.pGy
N189A d.gy.G
N189B d.gy.G/gy.G
N189C gGy
N189D l.gGy
N199A m.OlBr
N199B d.gy.yBr
N199C m.yBr
N199D s.yBr
N200A d.gy.yBr
N200B brGy
N200C l.brGy
N200D l.Gy

Despite the inclusion of color names that may not be appropriate for some of the chips on their respective leaves, the mini provides an excellent selection of color chips from the RHS Colour Chart (2001). The mixing of hues on certain leaves may be unusual, but for evaluation of plant colors - it works! Time will tell whether horticulturists find the 244 chips of the mini an adequate substitute for the 884-chip RHS Colour Chart. The smaller number of chips inevitably reduces the likelihood of finding a chip that closely approximates the color of a specimen. Thus, descriptions based on color evaluation with the mini will tend to identify specimen color as lying between two chips; for example, N57A - 61C. For many, the lower list price of the mini (£25 [$50]) vs. the RHS Colour Chart (£170 [$340]) will mean access to a high-quality color chart, thus benefiting the gardening community through better description of colors.

Don Voss, a member of the Potomac Valley Chapter, edited A Contribution Toward Standardization of Color Names in Horticulture.