In the London Philatelist of December 2020 (LP Vol 129, No 1481, p544), Ray Harris published an article titled 'Extending the colour key', in which he described a method for identifying the colour and tint of a stamp, or paper, using the Stanley Gibbons Stamp Colour Key, a scanner, suitable software and a Spreadsheet of his own devising, The article descries the methodology, and this page gives a link to the Spreadsheet Ray developed.

The Gibbons Colour Key is not perfect (see Stampboards discussion) but is very widely used. Ray's approach to colour identification required some work to calibrate the Spreadsheet, in order to ensure that the results from the user's own scanner tally with the 200 defined colours shown on the Gibbons Colour Key. Once callibated, and using the same scanner, the RGB colour values for the unknown stamp or paper are entered into the Spreadsheet, which returns the closest colour and tint (the shade or intensity of the basic colour).

LP Colour Matching with a Scanner Spreadsheet

The Spreadsheet contains instructions for callibration and use. Users are reminded to delete the RGB entries for the brown/neutral colours on the worksheet "originals & tints" in columns B-D, and then to enter data for each colour in the Key using their own scanner. It is also worth checking a few colours on the Colour Key from time to time to be sure that the scanner is remaining consistent. On some scanners the light will change over time.

Caveats with this method, and on using IT to measure colour, are that:

1. The colours used in the Gibbons catalogue to describe older stamps were judged by eye, using a colour card with 100 colours, so may not correspond exactly to results using Ray's method
2. Because blocks of colour on a stamp are rarely uniform it is important to follow Ray's instructions in the LP article for ensuring that the results come from a representative block of colour
3. Computer screens and monitors also present colour differently. Judging a colour based solely on what the screen shows is likely to lead to inaccuracy
4. Other 'scientific' approaches to the measurement of colour (for example Herendeen, David L., Allen, James A. and Lera, Thomas, 'Philatelic Shade Discrimination Based on Measured Colour', London Philatelist 120:1384 (April), pp105-117 rely upon spectroscopic measurements and statistical analysis
5. There's a whole world of work available online about colour and its measurement: fascinating but complex! It's all very well to analyse wavelengths but what we call colour is a result of a psycho-physiological perception rather than an independent physical phenomenon: a consequence of the stimulation of the human visual system by visible light.

Ray's article concludes:

This approach is proving an effective method for extending the range of the Colour Key to paler versions of colours, whether for stamps or postal papers. For plain papers and stamps with blocks of colour the choice of sample is easy, but for many stamps a choice of area and scanning resolution will need to be made. If stamp margins have plate numbers, they will certainly be in a single colour, but resolution at 600dpi or higher is likely to be necessary. When scanning the Colour Key, 200dpi or 300dpi will be sufficient. For the reason noted above, even pristine mint stamps may be assessed by this method to have a colour which varies from the catalogue. It is obvious that if the background is a coloured paper the result is unlikely to match exactly the colour of the printing ink that was used, whether assessing colours by eye or by scanning. However, the catalogue editor is similarly aiming to describe how the eye perceives a colour rather than to give an exact description of the ink, which is unlikely to concern most philatelists.