Since I had nothing better to do right now, I did a quick histogram check of the two green gems and the yellow gem in Gimp. Then you can read out average color value for the whole picture, as well as for each of the three colors red, green and blue.
I did my best to select only the main gems in the picture (the dark version), deselecting the outer round frame that had a black/blackish color.
I show the average numbers for the three gems; first all colors, then red, green and blue. 255 means the color is white, 0 means black.
COLOR - ALL RED GREEN BLUE
Normal Green - 159 71 159 36
St. Patrick - 175 88 175 43
Yellow - 220 220 175 64
From the ALL column, we see that St. Patrick is closer to the yellow overall. We see the same effect in all 3 color bands, being 10%-20% higher (brighter) in the St. Patrick version compared to the normal version.
Even though there shouldn't normally be any difficulties seeing a clear difference when only 2 gems are lined up beside each other, it is a different question when there are many of them, and the brain tries to get a quick overview of different patterns. As more difference there is in the colors, as easier and faster will the brain interpret the patterns.
The quick image analysis shows that the difference between St. Patrick Green and Yellow is smaller than the difference between Normal Green and Yellow.
Hence it supports the claim that the St. Patrick Green gems are harder than Normal Green Gems to differentiate from the yellow gems.