Not to be picky, but technically ODST's don't have medics. Because they're Marines they have corpsmen. Not a huge difference, but definitely one that an actual corpsman would point out. As for light/heavy/normal armor for a corpsman, I would think that it would just be normal. In a squad, every member has his or her own specific job. In H3: ODST we see this with Mickey being the demo expert, Romeo being the designated marksman, etc. And while they have slightly different armor permutations because of this, the basic ODST armor package is still there. So, I would imagine an ODST corpsman having the basic armor but with a slightly modified rucksack and maybe a medical pouch on a leg-drop platform. As for markings, I'd go with a red cross on the shoulder that doesn't have the "UNSC MED REF" tag and the surgeon symbol on the chest. Just my personal opinion.
As for color, I think that's totally up to the operator. I'm not sure where the "leaders wear red" came from seeing as in H3, H3: ODST, and Reach anyone can wear red regardless of rank. Also, Buck doesn't use red as his color and Mickey does. So, if you want your coloring to be red, go for it.