Determining precise colours on the classic issues of Nepal

As a collector of the classic issues of Nepal from 1881-1903 determining the many colours of each of the four denominations is quite difficult. Even on a single sheet the colours vary, yet the authors of the texts claim a vast number of shades for each value as the print runs were so small and spread over 22 years and double that if you include the vast number of reprints used to pay the telegraphic rates unto 1929. While the now out of print Stanley Gibbons Stamp Colour Key 2001 is useful it is printed in solid colours which are difficult to match on issues that are etched or engraved and appear of faded tones of the colour patches. SG did produce a Stamp Colour Guide in 1947 also out of print that used an image of each colour as a solid patch as well as an etched image, so there was a reliable tone reduction, but finding copies of that guide that are in pristine condition are rare. Now here is the challenge: There are several digital colour sensors on the market at present (I have a Nix Mini 2 sensor) and they have vast arrays of commercial paint colours in there reference sets and also define digital colours as used by printers, but is there a reference that illustrates or identifies colours by name generically, such as Ultramarine, Prussian Blue, green-blue etc.? I'd love to get a response that aids my curiosity.

from greenfingers in Australia