I own them and of course sell them to people all the time. The colours are fairly accurate, but keep in mind that flat gems don't have the faceted cone on the back therefore the layer of stone is thinner therefore the gem discs are slightly paler than the colour chart. I've seen them in pink, sapphire, siam, cz, aquamarine, jet, topaz, emerald and amethyst in real life, they are all very nice.
2mm is very popular for labret piercings (in their many incarnations), facial micordermals and microdermal projects. 4mm is mostly used on surface piercings and microdermals but also sometimes for labrets. We have also used the flat gems on stretched nostril piercings.