The Gemological Institute of America (GIA) used to offer plastic color masters for color grading. They argued that the plastic “gems” gave a more realistic representation of color than any flat media could. Unfortunately, the company making the color masters (Pantone) has gone out of business and so they are no longer available.
They made several different sets – the most common seems to be this one below, which is the one that I have, and I love it. I think I bought it for around $350 in late 2014 from a retiring jeweler, who had it from his GIA coursework in 1992. This image is not my exact one (although mine is identical). I found the picture of this one on an old eBay listing.
Here it is in practical use.
As an alternative, the GIA is now using a software color grading system invented by Israeli gem dealer Menahem Sevdermish. Students and dealers can download depictions of the color of the gemstones. The Gemewizard software identifies 31 master color hues, each of which is recreated in six color tones, which are subsequently each divided into six levels of saturation. All told, 1,116 gemstone colors are available through the system. This approach has been suggested before, but the primary obstacle is that computer monitors vary in their reproduction of colors.