I think they'd be more useful as perhaps a ring embedded in the lens or something (indicating health issues with a red ring in your field of view is well established in recent video games, after all), but it's a nice solution nonetheless.
Nanoparticles -- is there anything they can't do? -- embedded in the hydrogel lenses react with glucose molecules in naturally occurring tears. A chemical reaction then causes the lenses to shift their hues, alerting the wearer to falling or spiking blood sugar levels. The wearer can then make the appropriate adjustments to his or her blood sugar, all without having to carry around (and use) devices for drawing and analyzing blood.