The Royal Horticultural Society Colour Chart 2001
Donald H. Voss
Vienna, Virginia

The 2001 edition of the R.H.S. Colour Chart is notable for the substitution of paint-coated leaves for the eleven screen-printed leaves (57, 66, 74, 78, 80, 81, 82, 87, 88, 89, 109) used in earlier editions. The letter "N" is now prefixed to the leaf number for the eleven leaves that were formerly printed with the inks progressively more openly screened to achieve the lighter ("let-down") colors on each leaf. Use of this technique led to quality-control problems, and for some of the leaves large color differences occurred between individual sets of the chart, as well as between the 1966, 1986, and 1995 editions (Voss & Hale, 1998). The deposition of paints provides a means for achieving greater uniformity between sets and editions.

Using a Minolta CM-2002 spectrophotometer, the author compared a sample group of the screen-printed "A" chips from the 1995 chart (leaves 57, 66, 74, 78, 80, and 81) with their paint-coated replacements in the 2001 chart. The selected chips were closely comparable in terms of CIELAB lightness and hue angle (CIE, 1978). ("Hue angle" relates to the position of a specific color in a color wheel, a circular arrangement of hues: purplish red at 0°, yellow at 90°, bluish green at 180°, and blue at 270°.) The chips in the sample group from the 2001 edition, particularly in the Purple range, are somewhat lower in chroma (color saturation or purity of color) than the corresponding chips in the 1995 chart. (This result is to be expected: printing inks were used for the eleven screen-printed leaves in the earlier editions to obtain chromas higher than those available from paint mixtures.)

Nineteen new leaves were added to the chart in order to facilitate evaluation of specimen colors not adequately represented in the older editions. Thus, the total number of chips in the chart has increased from the 808 of earlier editions to 884. Each of the newly added leaves is inserted following a leaf with related colors - and, prefixed by "N," carries the same number! Thus, for example, there are now leaves numbered 25 and N25.

The 2001 chart is attractively packaged, but certain of its physical characteristics do not contribute to accurate evaluation of specimen color. The best results in visual color evaluation are obtained when the color chip and specimen being compared are viewed against a neutral background of approximately the same lightness as the specimen. The card stock used for the leaves of the R.H.S. chart is, however, a brilliant white that is highly fluorescent under ultraviolet radiation.

Another condition for accurate visual evaluation of color is exclusion from the field of view of colors other than those of the specimen and the color chip being compared. The colors of the upper covers of the first three fans are high in chroma and, because the fans are permanently riveted (instead of being assembled with screw posts), it is hard to keep these covers out of sight when using the chart. (Also, the tightness of the riveting in some sets makes it difficult to fan out the leaves when evaluating specimen color - the very purpose of assembling the chart in "fans"!)

The sequence of hues in the R.H.S. Colour Chart starts in Fan 1 with greenish Yellow and proceeds through Yellow, Orange, Red, Purple, Violet, Blue, Green, and back to greenish Yellow at the end of Fan 3. (Fan 4 comprises grayed colors of various hues.) Each leaf of the R.H.S. chart carries a "full color" chip marked as "A" and three successively lighter letdown chips marked "B," "C," and "D."

The progression of R.H.S. chart leaves around the hue circle is somewhat irregular in places, depending on whether one compares A, B, C, or D chips. Judging by the hues of their A chips, several of the nineteen newly added (i.e., not replacement) "N" leaves seem to have been inserted out of logical sequence. If, however, the let-down hues on these leaves are considered as the basis for insertion, the numbering found in the chart may be appropriate.

Because making the instinctive assumption that the chip colors on a given leaf of the R.H.S. chart are of the same hue can lead to error in visual color evaluation, users of the R.H.S. chart need to understand why substantial hue shifts are present on some leaves of the chart. The colors included in the 1966 R.H.S Colour Chart were chosen subjectively to maximize the chart's usefulness in evaluating plant colors. The chart was not based on a systematic sampling of a defined color space, and hue variation was present between the chips on individual leaves. One cause of such variation is addition to the A-chip colorant of white pigment in larger and larger amounts to obtain let-down colors. Intended to replicate the 1966 chart, the later editions also show let-down hue variations.

Combined with the greater lightness and lower chroma of the let-down chips, a few degrees of difference in hue angle between chips on an individual leaf will probably not be noticed by most users of the chart. At some point, however, hue angle differences between chips on a leaf become noticeable and complicate the process of evaluating specimen color.

Differences of 8 or 9 degrees of hue angle are generally obvious. Arbitrarily specifying a 9-degree difference between two chips on a given leaf of the R.H.S. chart as a threshold, 9 of the 59 leaves in Fan 1 and 27 of the 57 leaves in Fan 2 display noticeable differences; twelve of the leaves in these fans exhibit hue-angle differences of 18 degrees or more.

Nearly all of the differences of 9 degrees or more stand out clearly upon visual examination of the chart. Hue differences between chips on some leaves may be characterized as startling; several of these occur in the reddish Purple to Purple color range so important in evaluating rhododendron and azalea flower colors.

The Bottom Line
The leaflet accompanying the 2001 edition of the R.H.S. chart does not discuss the relation of hues on individual leaves. The instructions simply direct the user to find the chip that most closely approximates the color of the specimen. In conducting visual color evaluation with the R.H.S. color chart, it is important to take account of inconsistent hue relationships between A-chips and their respective let-down colors and then to adopt an appropriate procedure.

• A procedure appropriate for use with the R.H.S. chart is to expose leaves with chips of approximately the same lightness and hue range as the specimen and then identify the chip closest in color to the specimen (or, in many cases, two chips that bracket the specimen color). It is important to remember to check Fan IV of the chart for relevant colors.
• A method to avoid - though sometimes recommended (but not in the instructions accompanying the R.H.S. chart) - is to first locate the leaf with the A chip that most closely approximates the hue of the specimen and then select the chip on that leaf closest in color to the specimen. With the large hue-angle differences between chips on some individual leaves, this procedure can lead to confusion and possibly to error.

Because of color differences between some corresponding color chips in different editions of the R.H.S. Colour Chart, it is important to include identification of the edition when publishing color evaluations based on the chart. For example, if using the 1995 edition, prefix the selected chip identifier with "RHS95". Based on sets of the chart seen by the author, the four editions can be identified by their packaging:

RHS66 — cardboard box with removable top; dark grayish greenish blue label

RHS86 — silver-gray plastic flip-top box; produced in association with the Flower Council of Holland

RHS95 — dark green plastic flip-top box

RHS01 — red and black plastic flip-top box

The leaves of the 1986, 1995, and 2001 editions have a hole (about 1.2 cm in diameter) in each chip to facilitate evaluation of specimen color.

The Royal Horticultural Society Colour Chart remains a valuable tool for those evaluating plant colors and for those using published descriptions to visualize the colors. For some flower colors - especially light, high-chroma colors - the R.H.S. chart provides chips not available in some larger, higher-priced charts. For many genera, including Rhododendron, an additional consideration is that large numbers of cultivar descriptions recorded over the past half century have included color notations based on the R.H.S. chart.

References
CIE: Commission Internationale de l'Eclairage (International Commission on Illumination). 1978. Supplement no. 2 to CIE publication no. 15 (E-1.3.1) 1971/(TC-1.3). Paris.
The Royal Horticultural Society Colour Chart. 2001. London. (Available from R.H.S. Enterprises, Ltd., The R.H.S. Garden, Wisley, Woking, Surrey GU23 6QB, ENGLAND. [£135 in UK funds; equivalent to about US $200]
Voss, D.H. & Hale, W.N. 1998. A Comparison of the Three Editions of the Royal Horticultural Society Colour Chart. HortScience 33(1):13-17. [Brief extract in Journal American Rhododendron Society 52(3):132]

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